This story appears in the June 2017 issue of National Geographic magazine.

Veteran cavers Peter and Ann Bosted were cruising around their hometown of Hawaiian Ocean View, on Hawaii’s Big Island, a few years ago when Ann spotted a small hole off the side of the road.

It was no more than three feet wide—just big and inviting enough for the couple to pull their car over and try to slink into.

“We had a couple hours to kill,” Peter told me, “so we started surveying, and we found a side passage that turned out to be a lot more mazy than we expected.” Back home, Peter marked the puka, or cave entrance, on a digital map and planned to return later—with the landowner’s permission—to see where the opening might lead.

View Images Author Joshua Foer explores Kazumura, the world’s longest mapped lava tube, which stretches more than 40 miles and in places is the size of a subway tunnel. Its grooved walls took shape in the wake of an eruption approximately 600 years ago.

From outer space, the town of Hawaiian Ocean View looks like a thatched mat of asphalt draped over the side of Mauna Loa volcano. The 102-square-mile grid of crisscrossing streets and vacant lots is nearly twice the size of Washington, D.C., yet is home to fewer than 4,500 residents. You might think that only a pathological optimist would choose to build a house on the parched slope of an active volcano, but over the past two decades, Ocean View has become an international destination for cavers, who have come to explore and map the Kipuka Kanohina, a network of lava caves that course like veins 15 to 80 feet beneath the town.

There are two ways to make a cave: fast and slow. Many of the world’s most iconic caves—Carlsbad Caverns and Lechuguilla in New Mexico, Mammoth Cave in Kentucky—were carved out over millions of years, by the plodding drip and flow of acidic water through soluble limestone.

View Images Cavers climb a hillside to find an access point to the 26-mile-long lava tube called Kipuka Kanohina. As parts of the lava tube erode and crack over time, invading tree roots siphon moisture that collects, and forests thrive on the surface above.

By contrast, lava caves, widely known as lava tubes, are formed in a geological instant—a year or two, sometimes weeks—by an eruption from the Earth’s crust.

Most of Hawaii’s lava tubes are formed by a type of syrupy flow called pahoehoe. As it pours down the volcano, the lava at the surface is cooled by the air and solidifies, creating an elastic, skinlike outer layer. Beneath this inflating membrane, the lava continues to ooze, eroding the ground beneath it and carving underground tunnels. Now insulated from the air, the hot lava can surge unimpeded, often for many miles. As the eruption subsides and the channels drain their last molten contents, what’s left behind is a massive, 3-D fun house of plumbing.

Kauai Niihau Oahu Molokai Hawaii (U.S.) Maui Lanai U.S. 150 mi Hawaii 150 km Kohala 5,480 ft 1,670 m Waimea HawaiI Mauna Kea Hualalai 13,796 ft 4,205 m Hilo 8,271 ft 2,521 m Kazumura cave Kailua- Kona Mauna Loa 13,677 ft 4,169 m Kilauea 4,091 ft 1,247 m Puu Oo cone Hawaii Volcanoes National Park Hawaiian Ocean View Lava flow since 1800 Volcano Kipuka Kanohina caves 20 mi 20 km Of the Big Island’s five volcanoes, only Kilauea, Mauna Loa, and Hualalai have erupted since 1800. Kilauea’s Puu Oo eruption has been ongoing since 1983. MATTHEW W. CHWASTYK, NGM STAFF. SOURCES: USGS, HAWAIIAN VOLCANO OBSERVATORY; NATIONAL PARK SERVICE

Probably no other place on Earth has as many accessible lava tubes as Hawaii, and probably no other town has proved such fertile terrain for their exploration as Ocean View.

In the 1990s, the Bosteds were active members of the team that mapped the 138-mile-long Lechuguilla Cave, widely regarded as one of the world’s most beautiful. Now in their 60s and semiretired—Peter is a particle physicist affiliated with the College of William and Mary—they are among a handful of experienced cavers who have become full-time residents of Ocean View. Ann has pigtails down to her waist, and Peter sports a bright Hawaiian-print shirt, a white driver’s cap, flip-flops, and a biblical-length white beard. They say they’re now doing more caving than at any other point in their lives. They reckon that some years the two of them have spent more than 200 days underground.

View Images Peter Bosted crawls through a narrow lava tube in Kipuka Kanohina. His wife, Ann, follows behind. Thick gloves and clothes protect their skin from the sharp hardened lava. About three feet tall, this section of tube is positively spacious compared with what’s ahead—some spots are so narrow that the cavers must remove their helmets and shimmy through.

Peter and Ann brought me back to explore the new roadside puka they’d discovered, along with another couple, Don and Barb Coons, Illinois grain farmers and lifelong cavers who winter in Ocean View. Don, 64, was a guide at Mammoth Cave for 10 years and spent 18 winters on the legendary expeditions that helped expand the map of Chevé in Oaxaca, Mexico, the second deepest cave in North America. He’s the president of the Cave Conservancy of Hawaii, a nonprofit trust that has been buying up land in and around Ocean View to preserve the tunnels that lie beneath.

Wearing helmets, headlamps, and volleyball pads on our elbows and knees, we slither on our backs into the hole and army crawl for about a hundred yards through a previously mapped passage less than three feet tall. It has been centuries since lava flowed through this particular cave.

View Images Caver Chris Heinlein stands in Kazumura on the remnants of a lava pond. It was created when gas-filled lava pooled in one spot and then shrank as it cooled. Kazumura lies beneath the eastern slope of Kilauea, the most recently active volcano on Hawaii’s Big Island.

Festooned with trippy Dr. Seuss–like ornaments, the lava tubes of Hawaii seem to belong on another planet. Delicate lavacicles hang from the walls and ceilings like stalactites and take on a panoply of weird shapes, from spiky shark’s teeth to bubbly, gooey driblet spires. Long, hollow soda straws, squeezed out of the ceiling by gas while the cave was cooling, hang in thick clusters. In spots, the cave’s silvery, magnesioferrite glaze crinkles up like peeling paint. Elsewhere a thin layer of gypsum colors the walls a bright white, and mats of rock-eating bacteria excrete blue-green splotches of microbial poop.

Our army crawl ends at a junction where the ceiling dips down to less than a foot above the sharp, serrated floor. “This is our idea of fun,” Peter says drily, as we wriggle forward on our bellies into the impossibly tight crawlway, my T-shirt audibly ripping on the jagged floor. The passage is too cramped for even our helmets to squeeze through, so we take them off and shimmy forward in the dark.

View Images A choose-your-own-adventure section of Kipuka Kanohina splits into three tunnels, which will narrow as cavers go deeper. Navigating lava rock can be perilous. “The texture,” says one caver, “is like Velcro,” capable of tearing clothing and cutting skin. There are also dangers overhead: Chunks of sharp lava can break off and fall from the ceiling.

Forged By Fire Hawaii’s basaltic shield volcanoes have produced some of the world’s deepest and longest lava tubes—tunnels carved by rivers of molten rock. Caves etched by flowing water can take millions of years to form, while a volcanic eruption can generate miles of lava tubes in a matter of weeks or months. Thousands of these tubes twist, braid, and intersect beneath the surface of Hawaii’s Big Island. Forged By Fire Hawaii’s basaltic shield volcanoes have produced some of the world’s deepest and longest lava tubes—tunnels carved by rivers of molten rock. Caves etched by flowing water can take millions of years to form, while a volcanic eruption can generate miles of lava tubes in a matter of weeks or months. Thousands of these tubes twist, braid, and intersect beneath the surface of Hawaii’s Big Island. How A Lava Tube Is Formed Drag to explore Drag to explore A Volcano Erupts Lava Flows Lava Is Insulated A Cave Takes Shape Lava Escapes New Land Is Made Changing lava level inflated sheets Skylight covered Channels Flow ledge Original ground level How a LAVA TUBE is formed A Volcano Erupts Lava Flows lava is insulated a cave takes shape Lava Escapes New Lava is Made Molten rock and gas escape from deep beneath the Earth’s surface, sending lava spewing out in effusive and sometimes explosive eruptions. Factors like speed, gas content, and volume can yield two flow types: the rough and chunky aa or the smooth pahoehoe. Most lava tubes in Hawaii are formed by pahoehoe flows. As the outer layers of lava cool and solidify, they form an insulated tube, trapping heat and allowing the molten core to flow long distances. The volume of lava, speed of flow, and rate of erosion determine the nascent cave’s depth, ceiling height, and shape. If the lava volume increases or debris blocks the tube, lava can push up through open “skylights” or burst through cracks, making new surface flows or tubes. Since Kilauea’s ongoing Puu Oo eruption began in 1983, hundreds of acres have been added to the southeast coast of Hawaii’s Big Island. covered Channels inflated sheets As the slower moving sides of a lava channel gradually cool, solidify, and grow inward, the top of the flow can crust over, making a roof. On flatter slopes, pahoehoe flows can spread by inflat­ing and pushing forward sheets of lava while maintaining a molten core. Changing lava level inflated sheets Skylight Tubes are often built atop previous lava flows, channels, and tubes—the new flow burying the old. covered Channels Flow ledge Floating chunks of hardened lava can gather into a logjam, which helps form a crust. Extreme heat can melt the solid lava at the tube’s edges and base, enlarging the tube. If lava flows into the ocean, it can create gravelly and unstable spits of land in danger of collapse. As the top of the tube cools, cracks can form in the ceiling, causing portions to cave in. Original ground level A Volcano Erupts Molten rock and gas escape from deep beneath the Earth’s surface, sending lava spewing out in effusive and sometimes explosive eruptions. Lava Flows Factors like speed, gas content, and volume can yield two flow types: the rough and chunky aa or the smooth pahoehoe. Most lava tubes in Hawaii are formed by pahoehoe flows. Floating chunks of hardened lava can gather into a logjam, which helps form a crust. covered Channels As the slower moving sides of a lava channel gradually cool, solidify, and grow inward, the top of the flow can crust over, making a roof. inflated sheets On flatter slopes, pahoehoe flows can spread by inflat­ing and pushing forward sheets of lava while maintaining a molten core. Lava Is Insulated As the outer layers of lava cool and solidify, they form an insulated tube, trapping heat and allowing the molten core to flow long distances. Tubes are often built atop previous lava flows, chanels, and tubes—the new flow burying the old. A Cave Takes Shape The volume of lava, speed of flow, and rate of erosion determine the nascent cave’s depth, ceiling height, and shape. Extreme heat can melt the solid lava at the tube’s edges and base, enlarging the tube. Lava Escapes If the lava volume increases or debris blocks the tube, lava can push up through open “skylights” or burst through cracks, making new surface flows or tubes. As the top of the tube cools, cracks can form on the ceiling, causing portions to cave in. New Land Is Made Since Kilauea’s ongoing Puu Oo eruption began in 1983, hundreds of acres have been added to the southeast coast of Hawaii’s Big Island. If lava flows into the ocean, it can create gravely and unstable spits of land in danger of collapse. TUBES TO CAVES After an eruption subsides, the remaining lava hardens, and some tubes can become accessible caves. Many have long served as an important part of native Hawaiian culture and as a source of water and shelter. Drag to explore Drag to explore Branching Caves Secondary Formations Cave Dimensions Stalactites Skylight Lava-level marking Flow ledge Skylight Stalagmites tubes to caveS Secondary Formations Branching Caves Cave Dimensions After an eruption subsides, the remaining lava hardens, and some tubes can become accessible caves. Many have long served as an important part of native Hawaiian culture and as a source of water and shelter. Flow ledges, stalactites, stalagmites, lava-level markings, and shiny or textured walls can occur from changes in lava levels and cooling rates. Lava flows can split off into branching tubes and form vast, complex networks of caves. Some lava caves can be the size of subway tunnels, while others are too small for a person to squeeze through. Stalactites Skylight Lava-level marking Flow ledge Skylight Stalagmites Secondary Formations Flow lodges, stalacites, stalagmites, lava-level markings, and shiny or textured walls can concur from changes in lava levels and cooling rates. Branching caves Lava flows can split off into branching tubes and form vast, complex networks of caves. Cave Dimensions Some lava caves can be the size of subway tunnels, while others are too small for a person to squeeze through. Manuel Canales, Daisy Chung, Eve Conant, ngm staff; amanda hobbsart: TomÁŠ MÜllerSOURCES: Don Coons and michael warner, cave conservacy of hawaii

For all our scrapes, bruises, and torn clothing, our compensation this morning will be 154.4 feet of fresh cave added to the map of the Kipuka Kanohina network. That may not sound like much, but it’s through days like this that the map inches toward completion at the rate of three to four miles a year. Kanohina may soon be the longest surveyed lava tube system in the world.

The cave that the Kanohina system seems poised to supplant in the record books is on the other side of the Big Island. It was likely created during a 15th-century eruption of a different volcano, Kilauea. At more than 40 miles long, Kazumura is the longest lava tube mapped to date, and also the deepest. Though its roof is never more than a few dozen yards beneath the surface, the vertical drop—from the top of the cave, midway up the volcano, down to its terminus near the coast—is 3,613 feet.

Unlike the Kanohina system, which consists of several parallel passages that interweave like the delta of a large river, Kazumura is mostly one long, gaping straight shot of a tunnel—so wide and tall (more than 60 feet) in parts that it feels as if it could be easily adapted for a subway train. Despite Kazumura’s cavernous profile, the first through trip didn’t take place until 1995, when it was completed in a two-day expedition.

“This is a national treasure, and yet there are people on this island who live right on top of the cave and don’t even know it exists,” says Harry Shick, a landowner who operates tours through a stretch of Kazumura that lies under his property.

There is, it seems, an omertà that surrounds the Big Island’s lava tubes. Most cavers and conservationists would prefer that outsiders never learn the locations of their finds. When the Bosteds offered to take me to a cave called Manu Nui that they’ve been mapping since 2003, it was on the condition that National Geographic not reveal its precise whereabouts, except to say that it was created by Hualalai, the island’s third most historically active volcano, after Mauna Loa and Kilauea.

Manu Nui is, in many respects, the jewel of the island. With an average incline of 15.7 degrees, it is one of the steepest lava tubes in Hawaii, and its features are surreal. After entering the cave through a mist-shrouded puka on private land, we head uphill to a chamber that could be a fantasy from Willy Wonka’s Chocolate Factory. The walls, spattered in chocolate-, peanut butter-, cherry-, and butterscotch-colored drippings, look so luscious and fudgelike that I’m almost tempted to lick them. The Bosteds are keen to ensure the unique formations aren’t disturbed by curious adventure seekers. A lavacicle is a fragile thing, and it takes only one misplaced handhold to permanently disfigure a cave. Shick has meticulously gone through several miles of Kazumura, reattaching fallen features with superglue.

“We don’t even fully understand these cave ecosystems,” says Lyman Perry, from Hawaii’s Division of Forestry and Wildlife, “so we don’t want people going into them. The reality is that if people find out about these places, they’re going to ruin them eventually.”

Even more delicate than the caves’ features are the cultural sensitivities that surround them. Many native Hawaiians consider lava tubes kapu, or sacred sites, because of their frequent use as ancient burial grounds. In Hawaiian tradition, bones contain a person’s mana, or spiritual energy, and aren’t to be unnecessarily disturbed.

Keoni Alvarez, a 31-year-old activist and filmmaker who has battled developers trying to build atop burial caves, says that whenever human remains are found inside a lava tube, they render the entire cave system, start to finish, kapu. “We believe our caves are sacred and should not be desecrated,” he tells me. The problem is that no one can know whether a particular cave was used for ancient burials until it has been explored. Many native Hawaiians categorically refuse to venture into lava caves out of respect for what they might encounter inside.

View Images Pristine freshwater pools are rare in Hawaii’s lava tubes. They may look inviting, but explorers say divers can become disoriented in the twisting passages or trapped by blockages or rockfall and run out of air.

But if modern Hawaiians tend to be wary of lava tubes, their ancestors clearly used them quite a bit. Many cave openings show evidence of prehistoric habitation, complete with hearths and sleeping terraces. In war, longer lava tubes were sealed and used as “refuge caves” to hide women, children, and elders. In some cases, stone walls were built across tube entrances, leaving a passage just big enough for a single person to climb through.

A local expert estimates that one in two caves on the Big Island contains some sort of archaeological artifact. Especially on the dry, leeward side of the island, freshwater is hard to come by, and lava tubes were often the best place to find it. Deep inside Kanohina, hundreds of yards from entrances, one frequently comes across remains of kukui-nut torches and rings of rocks that once propped up gourds used to collect dripping water.

Don Coons and Peter Bosted are insistent about the difference between adventure and exploration. Adventure is what you do when you’re out for a thrill. Exploration is slow, methodical, and never for your sake alone. Every cave they explore, including this narrow, jagged section of the Kipuka Kanohina that we’re crawling through, must be meticulously surveyed and mapped using clinometers and laser range finders.

View Images Lava from Kilauea volcano pours out of tubes into the Pacific Ocean, its final destination. Much of this ever evolving landscape is a mystery: Researchers estimate that roughly 10 percent of Hawaii’s lava tubes have been mapped.

“The deep sea, outer space, and caves: Those are the only frontiers left,” says Coons, who does his exploring in a lightweight helmet with a small flashlight duct-taped to the brim. “On a workingman’s salary, you can go into an unexplored place and discover something new and be the only person in history to see it.”

Back in the roadside puka, our prone bodies pinched between the ceiling and floor, Bosted makes a disconcerting judgment call. “This seems a bit dangerous,” he says in his dry monotone. “I have to exhale in order to get through.” He announces that he’s turning around, leaving the rest of us to figure out where the cave might lead.

Seven and a half body lengths farther on, we reach a pile of breakdown rocks so heavy they can’t be budged from our prostrate position. The lead ends here for now, but the cool breeze we feel flowing over our faces can mean only one thing: There is more cave to chase on the other side.