This story appears in the September 2016 issue of National Geographic magazine.

“If you break loose here, you can’t stop. You’re going into the abyss,” barks Rich Rudow. Normally he is unflappable, but as he knows too well, this is no place to let down one’s guard. We’re on a cliff roughly 3,500 feet above the Colorado River at the tip of the Great Thumb Mesa, a spectacular formation that thrusts out from the South Rim of the Grand Canyon like the bow of an immense ship. It is one of the canyon’s most remote spots, rarely seen even by the most hard-core backpackers. If you come this far out on the Thumb, there is no way to get down to the river without climbing gear, and the dwindling food in your pack won’t allow you to make the eight-day trek back the way you came. You have to move forward.

Just ahead, the ledge that we’ve been walking on for the past several days vanishes into a deep indentation, or bay, in the wall of the canyon. This place is known as Owl Eyes, named for two enormous oval holes punched into the center of the cliff that looms over the middle of the bay. It’s a spooky place. Besides its ominous skull sockets, Owl Eyes is part of a tragic story. Nearly four years earlier, on a sunny February day, a beautiful young woman, a friend of Rudow’s, was crossing this passage when she fell to her death.

Now we’re staring across the same terrain, in far worse conditions. A storm had lumbered in the previous evening and coated the canyon in nine inches of snow. This is not what we’d imagined when we started this venture, an end-to-end hike of the Grand Canyon.

Escalade 0.5 mi (Proposed) 0.5 km 3,200-foot descent Tramway Navajo Nation reservation Chuar Butte Grand Direction of view above Canyon National Park Escalade (Proposed) 0.5 mi 0.5 km 3,200-foot descent Tramway Navajo Nation reservation Chuar Butte Direction of view above Grand Canyon National Park

It isn’t a particularly sane thing to attempt. There is no single trail or network of trails that stretches along the entirety of the North or South Rims. The most efficient way to travel the length of the canyon is to float down the Colorado River, which winds through the canyon for 277 sinuous miles. That’s why John Wesley Powell—who led the first documented traverse of the canyon—did so by boat.

After Powell’s achievement in the summer of 1869, more than a century would pass before the first known traverse by foot. During that time the canyon progressed from a forest reserve to a national monument until finally taking its place as the crown jewel of the National Park System and arguably the most recognized and beloved landscape in America. It became a vacation destination for hundreds of millions of families, its image captured on innumerable postcards. Yet nobody figured out how to walk all the way through the thing until a 25-year-old river guide named Kenton Grua completed it in the winter of 1976, some 65 years after both the North and South Poles had finally been reached, and 23 years after Mount Everest was first summited.

From River to Rim, Part 1: Pete McBride and Kevin Fedarko embark on a perilous adventure to make a sectional thru-hike through the Grand Canyon, a feat less than two dozen people have accomplished. Watch the team set out, uncertain of what lies ahead.

Think about that for a moment—and consider what it says about how complicated and wild this place truly is.

No one is sure of the exact distance Grua covered, but thanks to the countless bays, he probably walked more than 700 miles during his 37-day thru-hike along the south side of the river from Lees Ferry to the Grand Wash Cliffs.

He never publicized his feat. But as word of what he’d done slowly spread, a new challenge opened up to a tiny community of extreme backpackers, including an electrical engineer from Phoenix named Rich Rudow. By the autumn of 2015, Rudow had completed hundreds of hikes and slot canyon explorations in the canyon and felt he was ready for his biggest challenge: a 57-day trek moving east to west across the canyon’s north side.

By the time Rudow and two companions were ready to launch—almost 40 years after Grua’s thru-hike—fewer than two dozen people had approximated his feat by stringing together a chain of separate hikes along the length of the canyon, known as a “sectional” thru-hike. The number of trekkers who had completed a “continuous” thru-hike in a single push was even smaller. Before 2015 more people had stood on the moon (12) than had completed a continuous thru-hike of the Grand Canyon (eight).

When photographer Pete McBride heard about Rudow’s plans, he called him and asked whether we could join his group. Pete and I had years of experience boating in the canyon, but we were woefully unprepared for what lay ahead. The only explanation for Rudow’s agreeing is that he was swayed by our primary reason for wanting to do it: to look into disturbing reports we’d been hearing about the canyon’s future, which included new tourist developments, increased helicopter flights, and a uranium mine.

View Images Mathieu Brown (left), Kelly McGrath (center), and the author negotiate the Walter Powell Route to the South Rim.

Since it entered the American consciousness, the Grand Canyon has provoked two major reactions: the urge to protect it, and the temptation to make a whopping pile of money from it. During the years after the Powell expedition, miners rushed into the canyon to lay claims for copper, zinc, silver, and asbestos. During the 1880s one tycoon wanted to turn the bottom of the canyon into a railroad corridor to haul coal from Denver to California. (He drowned in the Colorado, along with two members of his survey expedition.) In the 1950s a mining company tried to get rich by building a giant cableway to move bat guano from a cave and sell it to rose gardeners; that didn’t last long. There was even a government plan to build a pair of giant hydroelectric dams in the heart of the canyon, a project that would have transformed large parts of the Colorado River into a series of reservoirs whose shorelines today would undoubtedly be clotted with houseboats and Jet Skis.

Contested Canyon Human activities may seem dwarfed by the 1,904 square miles of the Grand Canyon, but their impact could be wide reaching. Rising tourist numbers, air traffic, mining, and development have increasingly encroached on the park’s landscapes. Meanwhile the complex patchwork of federal, state, and tribal landownership complicates conservation efforts. NEVADA UTAH Grand Canyon National Park Direction of view ARIZONA Tourist Onslaught Grand Canyon National Park Skywalk 8 million of visitors a year Escalade (projected) 4 0 2013 2014 2015 Shortly after it became a national park in 1919, the Grand Canyon hosted some 37,000 visitors a year. Today about 5.5 million tourists arrive annually. It’s the second-most-visited national park, after the Great Smoky Mountains. Uranium Mines Active Standby Abandoned or reclaimed Claim Town water supplies Groundwater flow Surface Aquifer Commercial Development Existing Proposed Routes Helicopter tour route Paused Hiking route Resumed East Glen Canyon National Recreation Area Start of trek Page Lees Ferry Marble Canyon 20 The Trek Writer Kevin Fedarko and photographer Pete McBride are making a sectional thru-hike. They have completed seven sections of the canyon, with one more planned. Grand 89 Escalade Tramway Canyon Proposed Escalade road National Walter Powell Route Park Navajo Nation reservation KAIBAB NATIONAL FOREST 64 Down to Business Resort Tramway Empire State Building Colorado River The proposed tramway would allow visitors to descend 3,200 feet to a planned retail complex and food court overlooking the confluence of the Colorado and Little Colorado Rivers. South EAST Proposed monument boundary Grand Canyon National Horn Creek Park Orphan Tusayan development Grand Canyon N.P. KAIBAB NATIONAL FOREST Canyon Proposed monument boundary To Flagstaff 40 mi (64 km) 64 Polluted Water Retention pond Mine Water well 0 feet Colorado River Aquifer Ore 2,000 Impermeable layer 4,000 Contamination has occurred near older sites, including Hack Canyon and Orphan Mines, where erosion and problems with containment have allowed uranium to seep into waterways such as Horn Creek. Central Kaibab Indian Reservation Hermit Kanab North Pigeon Hack Canyon KAIBAB NATIONAL FOREST Arizona 1 Pinenut . Owl Eyes Supai HAVASUPAI Grand INDIAN Canyon RESERVATION National Park 385 Crowded Skies Helicopter flights near Quartermaster Canyon 204 Average number of flights over three days during peak season. 2011 2013 The Federal Aviation Administration caps air tours at 93,971 flights a year. This limit doesn’t apply to the Hualapai, who are free to run unlimited air tours from their land. The FAA doesn’t track Hualapai flights, and the tribe declined to say how many they allow a year. West Lake Mead National Recreation Grand Canyon- Parashant National Monument Area Planned end of trek Grand Skywalk GRAND CANYON WEST Quartermaster Canyon Canyon Quartermaster Canyon National Park Trek halted HUALAPAI INDIAN RESERVATION SCALE VARIES IN EACH PERSPECTIVE MAP. THE LINEAR DISTANCE BETWEEN THE GRAND CANYON ESCALADE SITE AND THE GRAND CANYON SKYWALK IS 114 MILES (183 KM). CHARLES PREPPERNAU AND MANUEL CANALES, NGM STAFF; AMANDA HOBBS. SOURCES: BUREAU OF LAND MANAGEMENT; COCONINO PLATEAU WATER ADVISORY COUNCIL AND WATERSHED PARTNERSHIP; DONALD BILLS, USGS; ERIC FRYE AND EDWARD SCHENK, NATIONAL PARK SERVICE; ESRI; GRAND CANYON ESCALADE; GRAND CANYON WEST; STEPHANIE SMITH AND ROGER CLARK, GRAND CANYON TRUST Contested Canyon Human activities may seem dwarfed by the 1,904 square miles of the Grand Canyon, but their impact could be wide reaching. Rising tourist numbers, air traffic, mining, and development have increasingly encroached on the park’s landscapes. Meanwhile the complex patchwork of federal, state, and tribal landownership complicates conservation efforts. NEVADA UTAH AREA ENLARGED Uranium Mines Town water supplies Routes Commercial Development Helicopter tour route Aquifer Surface Active Existing Proposed Direction of view ARIZONA Paused Groundwater flow Standby Glen Canyon National Recreation Area Hiking route Abandoned or reclaimed Kaibab Indian Reservation Resumed Claim Lake MEAD Start of trek Hermit Pigeon Grand Canyon- Parashant National Monument National Kanab North Hack Canyon Recreation KAIBAB NATIONAL FOREST Lees Ferry Area Arizona 1 20 Pinenut . Planned end of trek Lake MEAD Proposed monument boundary 89 Owl Eyes Chapel National Recreation Navajo Nation reservation Grand Grand Area Skywalk Ridenour Canyon Canyon National Escalade Tramway To Las Vegas 70 mi (113 km) Park National Horn Creek HUALAPAI INDIAN RESERVATION Walter Powell Route Orphan Park HAVASUPAI INDIAN RESERVATION Tusayan development 64 Trek halted KAIBAB NATIONAL FOREST To Flagstaff 40 mi (64 km) Canyon 64 SCALE VARIES IN THIS PERSPECTIVE. Proposed monument boundary THE LINEAR DISTANCE BETWEEN THE GRAND CANYON ESCALADE SITE AND THE GRAND CANYON SKYWALK IS 114 MILES (183 KM). Crowded Skies Polluted Water Tourist Onslaught Down to Business 385 Grand Canyon National Park Helicopter flights near Quartermaster Canyon Resort Retention pond Mine Water well Skywalk 0 feet 8 million visitors a year Escalade (projected) 204 Colorado River Tramway Aquifer Ore 2,000 Empire State Building 4 Average number of flights over three days during peak season. Impermeable layer 4,000 Colorado River 0 2011 2013 2013 2014 2015 The Federal Aviation Administration caps air tours at 93,971 flights a year. This limit doesn’t apply to the Hualapai, who are free to run unlimited air tours from their land. The FAA doesn’t track Hualapai flights, and the tribe declined to say how many they allow a year. Contamination has occurred near older sites, including Hack Canyon and Orphan Mines, where erosion and problems with containment have allowed uranium to seep into waterways such as Horn Creek. Shortly after it became a national park in 1919, the Grand Canyon hosted some 37,000 visitors a year. Today about 5.5 million tourists arrive annually. It’s the second-most-visited national park, after the Great Smoky Mountains. The proposed tramway would allow visitors to descend 3,200 feet to a planned retail complex and food court overlooking the confluence of the Colorado and Little Colorado Rivers. CHARLES PREPPERNAU AND MANUEL CANALES, NGM STAFF; AMANDA HOBBS. SOURCES: BUREAU OF LAND MANAGEMENT; COCONINO PLATEAU WATER ADVISORY COUNCIL AND WATERSHED PARTNERSHIP; DONALD BILLS, USGS; ERIC FRYE AND EDWARD SCHENK, NATIONAL PARK SERVICE; ESRI; GRAND CANYON ESCALADE; GRAND CANYON WEST; STEPHANIE SMITH AND ROGER CLARK, GRAND CANYON TRUST

The successful campaign to stop those dams, spearheaded by the Sierra Club during the 1960s, established the idea that the Grand Canyon is inviolable. And yet Pete and I had heard about a range of new proposals—many of them driven by savvy entrepreneurs operating just outside the canyon’s boundaries in areas that were controlled not by the National Park Service but by the U.S. Forest Service or one of the five Native American tribes whose federally recognized reservations are located around the canyon. From every point of the compass, threats ranging from colossal tourist developments and unlimited helicopter tours to uranium mining were poised to spoil one of the world’s premier parks.

It seemed to Pete and me that the best way to understand what was really at stake was to follow Kenton Grua’s example and hike straight through the heart of it all.

“Dude, are you all right?” Pete murmurs, shaking me gently. “Wanna try and eat something before you totally pass out?”

It’s late September, the sun is about to set on our first day of walking, and I’m splayed across the narrow patch of dirt where we’re supposed to spend the night.

One of the many things that I hadn’t prepared for is that there’s nothing gradual about this initial stretch of the journey. The canyon sucker punches its challengers with some of the most punishing territory right out of the gate. Add to that our 50-pound packs and an early autumn heat wave that pushed temperatures to 110 degrees, which wrung every bit of moisture out of our bodies and had begun peeling away the soles of our hiking shoes.

By the next morning Pete felt even worse than me. He had muscle cramps so intense that when he removed his shirt, it looked as if a mouse had wriggled into his abdomen and was scurrying from his shoulders to his stomach and back, just beneath the skin.

On day six we acknowledged that we were in over our heads and bailed, leaving Rudow and his partners to continue. On the trek out, Pete was delirious and disoriented, and once back in Flagstaff, he was diagnosed with hyponatremia, a heat-induced imbalance of salts and minerals, which, left untreated, could result in death.

In late October, intimidated but not defeated, we descended back into the now much cooler canyon and resumed our journey at the milepost where we’d pulled out three weeks earlier. Over the next several days, we threaded a route along a dizzying set of limestone ledges that dropped almost a thousand feet straight down to the river. Near river mile marker 32, we could discern the shadowy portal of the cave where archaeologists have found artifacts of the ancestral Puebloans, who inhabited this landscape for more than 10,000 years, as well as the remains of Harrington’s mountain goat (Oreamnos harringtoni) and yesterday’s camel (Camelops hesternus), now extinct creatures that flourished until the end of the Pleistocene, about 12,000 years ago.

A daily pattern emerged: Each morning we would stuff ourselves with oatmeal, then set out on a 12- to 14-mile slog that usually involved hauling our packs up as much as a thousand vertical feet, descending impossibly steep slopes, or pushing through thickets of thornbushes. This would go on until the sun began to set, at which point, battered, scratched, and bone-tired, we would boil water, wolf down some rehydrated dinner, then lie back and gaze at the night sky while listening to the words of Edward Abbey on an audiobook Pete had downloaded onto his phone.

The book was Desert Solitaire, Abbey’s homage to the country of the Grand Canyon’s sister parks, Canyonlands and Arches. Although I was usually too exhausted to stay awake for more than a few sentences, I often asked Pete to replay the part where Abbey warns readers not to jump into their cars next June and rush out, hoping to see some of the wonders he had attempted to evoke:

In the first place you can’t see anything from a car; you’ve got to get out of the goddamned contraption and walk, better yet crawl, on hands and knees, over the sandstone and through the thornbush and cactus. When traces of blood begin to mark your trail you’ll see something, maybe. Probably not.

From River to Rim, Part 2: Almost 200 miles into the journey, winter sets in. Facing frigid temperatures, snow-obscured terrain, and dwindling food, the crew soldiers through—right down into the depths of the magnificent Olo Canyon.

Although that passage seemed to speak most directly to me in the moment, I always willed myself to stay awake for what followed:

In the second place most of what I write about in this book is already gone or going under fast. This is not a travel guide but an elegy. A memorial. You’re holding a tombstone in your hands.

Those words, which Abbey wrote in 1967, carried a disturbing prescience because the wilderness of Arches that he once reveled in is now overwhelmed by so many visitors—1.4 million in 2015—that the entrance to the park had to be closed intermittently on Memorial Day weekend last year. And due to a dam project, the wonders of Glen Canyon, said to rival the beauty of the Grand Canyon, now lie beneath the surface of a 186-mile-long reservoir named after John Wesley Powell.

As Pete and I were about to discover, changes that bear a disturbing resemblance to the forces that Abbey had warned against—growth, development, and the pursuit of money—are unfolding inside Grand Canyon.

Sixty-two river miles downstream from Lees Ferry, the reddish brown Colorado encounters its largest tributary within the canyon, a river known as the Little Colorado, whose waters often run a brilliant shade of turquoise. The point where the two streams merge, known as the Confluence, holds profound spiritual significance for many Native Americans whose ancestral lands lie within the canyon, including the Havasupai, the Zuni, the Hopi, and the Navajo.

On the morning of November 2, we emerged on the north side of the river, inflated a pod of tiny rafts that we’d been carrying at the bottoms of our packs, and paddled across to begin an arduous 3,500-foot climb through a series of steep breaks in the cliffs that eventually delivered us to a remote stretch of the canyon’s eastern rim and the western border of the Navajo Reservation. We selected this route because it runs parallel to the path along which a group of developers from Scottsdale intend to construct the Escalade Tramway. Eight-person gondolas would shuttle tourists from the rim to near the river’s edge, where the developers plan to erect a retail complex, food court, and amphitheater overlooking the Confluence.

The tramway would be capable of delivering as many as 10,000 people a day to a spot that now rarely hosts more than a few dozen people on a typical summer day, and often none during the winter. There has never been a development like it inside the canyon.

The driving force behind this project is R. Lamar Whitmer, a political consultant who has persuaded a group of Navajo politicians that it would bring much needed revenue to the tribe. The opposition includes environmentalists as well as virtually every tribe in the region, including a group of Navajo who say that Whitmer and his associates tricked some tribespeople into supporting the project with misleading promises. (Whitmer denies he misled anyone.)

View Images A flight over the upper Grand Canyon offers a view of the edge of the Navajo Reservation (far side of the canyon). No single spot provides a view of the whole canyon. Its scale disguises its fragility, says Roger Clark of the Grand Canyon Trust. “Our greatest challenge is making people understand just how truly vulnerable this place actually is.”

This group calls itself Save the Confluence. When one of its members, Renae Yellowhorse, got word that Pete and I were scheduled to pop out of the canyon at a spot overlooking the Confluence, she telephoned a friend and asked him to drive her 41 miles from her home on the western edge of the Navajo Reservation, so that she could share a pot of traditional mutton stew and give us a piece of her mind.

According to Yellowhorse, the reservation was now abuzz with rumors that Whitmer and his allies were assembling investors to finance the billion-dollar project while simultaneously forging new alliances with Navajo legislators in the hopes of making an end run around Navajo president Russell Begaye, a prominent opponent of the project. “We’re not opposed to development, but it’s not appropriate here,” declared Yellowhorse, a fiercely determined woman in wire-rimmed glasses and leather moccasins. “When my grandchildren come, I want them to see this place the way that my ancestors saw it. We don’t want this area developed—we do not want to see Disneyland on the edge of the canyon.”

As it turned out, the friend who had driven Yellowhorse to meet us, a man named Roger Clark, was able to provide some context for that statement. As a program director of the Grand Canyon Trust, a conservation group that has spent 30 years battling an array of threats against the canyon, Clark is deeply troubled by the tramway plan. But he is even more worried that this project is part of a larger ring of threats that present an unprecedented assault on the integrity of the canyon.

At the Grand Canyon, summer brings heavy traffic. This sequence of images—a time-lapse composite of 160 choppers on two different flight paths—covers an eight-hour period on July 9, 2016. Besides numerous boats, 363 helicopters were counted. On peak days, that number can exceed 450.

One of the other issues that concern Clark and many other environmentalists is Tusayan, a small town composed of a strip of modest tourist motels and gas stations two miles from the park’s main entrance at the South Rim. Tusayan has been taken over by a consortium of investors who want to transform it into a resort, with potentially thousands of new homes and millions of square feet of commercial space, including luxury hotels, a European-style health spa, and a dude ranch.

All of this will require lots of water. The developers, led by an Italian company called Stilo, say they are reviewing ways to bring in water, including by train or a pipeline tapped into the Colorado River. But they also have the right to punch wells through the surface of the arid South Rim to access an aquifer that drives many of the springs and seeps deep within the Grand Canyon. These tiny pockets where water trickles from cracks in the bare rock make up less than 0.01 percent of the surface area inside the canyon, but each little oasis supports a web of complex plant and animal life. Thanks to the 6,000-foot elevation difference between the Colorado River and the North Rim, the canyon boasts five of North America’s seven “life zones”—more than any other national park. In latitudinal terms, it’s the equivalent of walking from the deserts of northern Mexico to the boreal regions of Canada, all in the span of little more than a vertical mile. Biologists say anything that might taint these springs or induce them to dry up would reverberate throughout the canyon’s biome.

Clark didn’t know it at the time, but the U.S. Forest Service would soon refuse to review the town’s application for a road easement that is crucial for the project to go forward. But Tusayan’s backers already have overcome many obstacles, and if they find a way to clear this final hurdle, little will stand in their way.

Tusayan, however, isn’t the only threat to the region’s aquifers. Just six miles to the southeast of the town—also outside the park—a company called Energy Fuels has reopened a mine after a bitter court fight with environmental groups and the Havasupai tribe and soon will be hauling out uranium ore. A company official dismissed the possibility of a major accident. But according to U.S. Geological Survey data, 15 springs and five wells inside the Grand Canyon area have levels of uranium that are considered unsafe to drink, due in part to incidents in older mines, where erosion and problems with containment have allowed uranium to leach into the groundwater.

From River to Rim, Part 3: As they make their way into remote western Grand Canyon—“the Godscape”—the team faces a constant, unrelenting fear: running out of water. Its scarcity poses a unifying challenge to all those who want to preserve and develop the park.

Meanwhile a 22-mile stretch of the river corridor at the bottom of the western end of the canyon has been opened to unlimited air traffic by the Hualapai, a tribe whose reservation borders the south side of the Colorado River. Thanks to a Federal Aviation Administration rule change requested by the Hualapai, the tribe may operate an unrestricted number of helicopter flights. These are filled with sightseers, many from Las Vegas, and fly below the canyon’s rim from sunrise to sunset. The noise they generate is so intense, and so continuous, that the area is locally known as Helicopter Alley.

“When you look across this vast landscape now, it’s hard to believe that it could possibly be damaged or lost due to acts of man,” Clark said. “But each of these threats is capable of eroding a piece of the canyon’s majesty, and together they will strip the landscape of its ability to do the thing that makes it unique, which is to instill humility by demonstrating that human beings are tiny in relation to the forces that have shaped this planet, and that we are not the center of the world.”

The bigger threat, Clark contends, is that Tusayan, the tramway, and Helicopter Alley have the potential to accelerate neighboring development projects. He noted that the Hualapai’s wildly successful helicopter operation has drawn interest among some Navajo, who believe that the cable-driven gondola system could be an anchor for a similar explosion of air tours along the eastern flanks of the canyon. If that vision were realized and if Tusayan’s development were to move forward, Clark said, the impact would be enormous. “You would have a mega-resort perched directly above the central portion of the canyon and bookended by a pair of massive air-tour operations, each anchored to its own new development,” he said. “In a very real sense, the entire sweep of the canyon would be transformed into something that looks less like a national park and more like an amusement park.”

After Thanksgiving Pete and I headed back to where we’d ended our previous push and began hiking downstream. One hundred twenty-two miles later, we climbed back out, at the park’s South Rim entrance. Next came a 66-mile push that began just after New Year’s. Our pace each day was determined by the location of springs, which we relied on for drinking water, hopscotching from one to the next. At a place called Horn Creek, we had to bypass a large spring contaminated by an abandoned uranium mine just below the South Rim that has poisoned its water since the 1960s.

At the end of January, as we were preparing for the most formidable leg of all—a 155-mile thrust around the Great Thumb Mesa—our friend Rich Rudow reentered the picture. He and his partner, Chris Atwood, had passed through the Grand Wash Cliffs in late November, becoming the ninth and 10th people to ever complete a continuous thru-hike of the entire canyon. (Their friend Dave Nally had pulled out early with respiratory problems.) Rudow had been tracking our progress via satellite texts we’d been sending and was worried about the challenges Pete and I would face on the Thumb in winter, when storms can blow in with little warning and dump several inches of snow.

Rudow had decided that he needed to return to shepherd us through. Which is how, on the afternoon of February 1, we all came to be standing in almost a foot of snow at the edge of Owl Eyes, wondering how we were going to make it across.

At the far end of the horseshoe-shaped bay was a massive ledge. If we could reach that flat piece of ground, we’d be OK. But getting there would require navigating directly across a steep slope of shale, hoping that if we slipped, we’d be able to stop our slide before shooting over the 400-foot cliff. It was already late in the afternoon, and if we failed to make it to safe ground before dark, we’d confront the appalling prospect of having to spend the night on the treacherously slick slopes of Owl Eyes.

After more than two hours, we’d only made it to the middle of the horseshoe, where a small promontory extended out from the slope. It was no more than 20 yards long, but there was a flat space on top, and at the far end there was a small pile of stones. When we reached the stones, Rudow halted and bowed his head for a moment. Then he removed his glasses and wiped his eyes.

“I’m so sorry,” he said softly. “It’s very emotional to be standing here.” Then he told us the story of what had happened to the young woman in whose memory the stones had been placed.

View Images Former Grand Canyon ranger Amy Martin watches a winter sunrise over a section of the Colorado called Conquistador Aisle. No trails exist along 95 percent of the canyon’s North Rim or along 80 percent of its South Rim. Fewer people have now hiked the canyon’s length in a single trip (10) than have walked on the moon (12).

Her name was Ioana Elise Hociota. She originally was from Romania, spoke four languages, and had degrees in mathematics and biology. She was 24, newly married, and she and her husband, Andrew Holycross, were close to completing a sectional thru-hike of the canyon.

By the winter of 2012, Hociota had set her sights on the 20-mile stretch of ledges near the Great Thumb Mesa. When Holycross realized that his work schedule would prevent him from going, Hociota paired up with Matthias Kawski, a math professor and her academic mentor.

They were in the middle of Owl Eyes when they stopped for lunch. Afterward Kawski headed farther up into the shale. Hociota opted for a more direct line that took her out of Kawski’s sight. A minute or two later Kawski heard a rock fall, followed by a sharp scream, and then, after a few seconds, a hollow thump. Scrambling to the edge of the cliff, he peered down, looking in vain for Hociota. He called out over and over. Nothing.

The next day Hociota’s body was discovered, and a ranger tethered to a helicopter was lowered to retrieve it. When Rudow finished his story, he looked west, where the sun was angling toward the canyon’s rim. “Guys,” he announced. “We’re gonna have to spend the night here.”

That night all of our water bottles froze, even though we stashed them inside the two tents that we pitched on the tiny patch of flat ground next to Hociota’s memorial. Our shoes froze too, and the next morning we had to hold them over our camp stoves to thaw them out.

We broke camp and trudged the rest of the way across the snow-encrusted slopes to the flat ledge on the far side of Owl Eyes, where we dried our gear in the sun and looked back at the ground we had crossed.

It was a sad and dangerous place, and I was glad to be done with it. But I couldn’t help but note that it was also quite beautiful. In the morning sunlight, even the face of the cliff down which Hociota had fallen was coated in a honey-colored glaze that seemed to glow from within. In that moment I may have glimpsed part of what Edward Abbey meant when he wrote about how it’s necessary to crawl across this terrain and bleed before you finally see something.

View Images Photographer Pete McBride and writer Kevin Fedarko make their way through Marble Canyon. The explorer John Wesley Powell gave the canyon its name, despite the fact that it contains no marble. “The limestone of this canyon is often polished, and makes a beautiful marble,” he wrote in 1869. “Sometimes the rocks are of many colors—white, gray, pink, and purple, with saffron tints.”

What I saw—or rather, what I understood—was that of the many things that had drawn a math prodigy from Romania into this landscape was that the canyon is emphatically not an amusement park. It is without handrails, a place where the dangers are real. But no less real are the rewards—among them the fact that when you move through an ancient wilderness that has not been compromised, you are reminded of our species’ humble place in it and the fragility of life. Apparently Ioana Hociota understood that she needed places like that. And I suspect that the rest of us may need them too.

Four days later we hiked out. And after resupplying in Flagstaff, Pete and I resumed our thru-hike in a series of pushes that, by the middle of March, brought us to within 50 miles of the end. But the canyon wasn’t through with us. One morning the thermometer on Pete’s watch hit 111 degrees, hotter than the temperature that had triggered his hyponatremia six months earlier. Thirty minutes later we started hiking out.

When we began this quest, we had no way of knowing that even after flinging ourselves at the canyon on seven separate trips over the course of a year, the end would still lie before us. As you’re reading this in September 2016, it’s likely we’re back on the trail, trying to finish our thru-hike. If you’re reading this story decades from now, say in 2066, hopefully a vast Grand Canyon wilderness, in the truest sense of the word, still exists.

Your National Geographic Society membership helped fund this expedition.

Watch the documentary Into the Grand Canyon on National Geographic, as PeteMcBride and Kevin Fedarko traverse the Grand Canyon by foot, hoping to better understand this revered landscapes and the threats poised to alter it forever.