In June 2010, Bill Ewasko traveled alone from his home in suburban Atlanta to Joshua Tree National Park, where he planned to hike for several days. Ewasko, 66, was an avid jogger, a Vietnam vet and a longtime fan of the desert West. A family photo of Ewasko standing at the summit of Mount San Jacinto, another popular hiking destination in Southern California, shows a cheerful man with a salt-and-pepper mustache, looking fit, prepared and perfectly comfortable in the outdoors.

Ewasko left a rough itinerary behind with his girlfriend, Mary Winston, featuring multiple destinations, both inside and outside the park. His first hike, on Thursday, June 24, was meant to be a loop out and back from a remote historic site known as Carey’s Castle, an old miner’s hut built into the rocks. Carey’s Castle is so archaeologically fragile that, to discourage visitors, the National Park Service does not include it on official maps. Winston, a retired mortgage broker, was worried about that particular hike. From what she had read, the site sounded too remote, too isolated. She so thoroughly pestered Ewasko about his safety that, when he arrived in California, he bought a can of pepper spray as a kind of reassuring joke. Don’t worry, Ewasko told her. He would be all right.

The plan was that after he finished the hike, probably no later than 5 p.m., he would call Winston to check in, then grab dinner in nearby Pioneertown. But 5 p.m. rolled around, and Ewasko hadn’t called. Winston tried his cellphone several times, and it went directly to voice mail. She knew he might still be in a region of the park with limited cellular access, but the thought was hardly reassuring. As night fell on the West Coast with no word from Ewasko, Winston tried to call someone at the park, but by then Joshua Tree headquarters had closed for the day. Her only option was to wait.

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The next morning at a little before 8 a.m., Winston finally got through to park rangers to explain her situation: Her boyfriend was missing, a solo hiker presumably lost somewhere in the precipitous terrain surrounding Carey’s Castle. Rangers went immediately to the trail head, but Ewasko’s rental car, a white 2007 Chrysler Sebring, was nowhere to be seen. What’s more, the trail appeared to have had no visitors for at least a week. Ewasko had apparently changed plans.

Joshua Tree is highly regarded among climbers for its challenging boulder fields, but its proximity to civilization and its tame outer appearance have given it a reputation as an easy destination — not the sort of place where a person can simply disappear. One of the most heavily trafficked national parks in the United States, Joshua Tree is only two hours from Los Angeles, a megacity whose regional population now exceeds 12 million. An hour’s drive southwest of the park is the irrigated sprawl of Greater Palm Springs, an air-conditioned oasis of luxury hotels and golf courses, known as much for its contemporary hedonism as for its celebrity past. It is this domesticated, unthreatening version of the desert that many visitors last see before driving into Joshua Tree’s wild interior.

“It looks kind of benign to a person who drives through it,” Dave Pylman told me. “But there are so many areas where you can get lost and not even realize it until you’re lost. You can’t look back and figure out, ‘Where did I come from?’ ” Pylman, 71, is a former executive director of Friends of Joshua Tree, a climbing-advocacy group, as well as a 19-year veteran of Joshua Tree Search and Rescue. The park contains “areas of unknown difficulty,” he said, where large rocks lean together, forming dangerous pits and caves; in other spots, apparently minor side canyons can take more than an hour to summit. The National Park Service also warns that the landscape hides at least 120 abandoned mine shafts into which an unsuspecting hiker might stumble.

Pylman’s involvement with the Ewasko case began soon after Winston’s call. As deputy planning chief, he was put in charge of routes, teams and search areas. At first, he said, Ewasko appeared to be a typical lost tourist: someone who goes out by himself, encounters a problem of some sort, fails to report back at a prearranged time and eventually finds his way back to known territory. The park sees nearly 50 such cases every year. Learning that Ewasko was a fit, accomplished hiker added to Pylman’s confidence that he would be found quickly and perhaps even “self-rescue” by finding his own way out.

Carey’s Castle was only one of several locations on Ewasko’s itinerary. Unfortunately, the list included sites as far-flung as the Salton Sea and Mount San Jacinto, each more than an hour’s drive from the park. Rangers quickly established that Ewasko’s National Parks pass had never been scanned at either park entrance. That wasn’t definitive proof of anything — if a long line of cars forms, members are often waved through — but it meant that there was no record of his visit. Had Ewasko even entered Joshua Tree?

A handful of other trails within the park also featured on his list. There was Keys View, an overlook with views of the San Andreas Fault, as well as the exposed summit of Quail Mountain, Joshua Tree’s highest point, part of a slow transition into the park’s mountainous western region. “The thing I remember the most,” Pylman said, “was the frustration of: How can this be? How can we have so much information about where he was going to go, or at least where he said he was going to go — why can’t we find him?”

Pete Carlson of the Riverside Mountain Rescue Unit indicating an area in Joshua Tree National Park where Ewasko may have wandered. Philip Montgomery for The New York Times

It was not until the afternoon of Saturday, June 26, nearly two full days after Ewasko failed to call Mary Winston, that a California Highway Patrol helicopter finally spotted Ewasko’s car at the Juniper Flats trail head, nearly a 90-minute drive from the Carey’s Castle trail head. By this time, he would have been exposed to late June temperatures hovering in the mid-90s, probably with little food or water. Locating the car did indicate that Ewasko was — or had at one point been — inside the park, and the rapidly expanding search effort immediately shifted to Juniper Flats.

Stretching west from Juniper Flats, where Ewasko’s car was spotted, is an old, unpaved road that begins with little promise of an eventful hike; chilling winds whip down from the flanks of Quail Mountain, and the park’s famous boulder fields are nowhere near. But as the dirt road continues, hikers are confronted by cascading decision points — places where the trail diverges at junctions with other trails or where it crosses a wash or dry streambed. As they compound over time, these minor decisions give rise to radically different situations: an exposed cliff instead of a secluded valley, say, or a rattlesnake-filled canyon instead of a quiet plain.

Anticipating what a stranger will do when confronted with decision points in an unfamiliar landscape is part of any search-and-rescue operation. In recent years, technology — in the form of what are called lost-person-behavior algorithms — has been brought to bear on the problem. Some of the most widely used algorithms are those developed by the Virginia-based search-and-rescue expert Robert Koester, who wrote the definitive book on the subject, “Lost Person Behavior.” “The basic premise,” Koester told me, “is that the past predicts the future. While you can never pinpoint exactly where you think the missing person you’re looking for is going to be located — if you could, it would be a rescue, not a search — by looking at enough previous cases that are similar, you can build a statistical model that identifies the most likely locations.”

As Koester explained to me, many lost hikers believe they are headed in the right direction until it’s too late. An animal trail that resembles a new branch of the path might divert downhill to a stream, for example, before winding onward through a series of ravines, ending at a dry wash — but by then an hour or more has gone by, and the path forward is now nowhere to be seen. Worse, Koester said, simply turning around can be impossible, as the route back is camouflaged by rocks or brush. The hiker is lost.

Koester has assembled a database of nearly 150,000 search-and-rescue cases. From these, he has produced a series of algorithmic tools that can be applied to future situations, helping to estimate not just where a lost person might be but also the sequence of decisions that led that person there. Still, it is a high-endurance detective operation. Well-trained searchers, he said, will perform methodical eye movements to allow themselves to take in the full visual field, scanning continuously for any abnormalities in the landscape — a footprint, broken branches, a discarded piece of clothing — that could suggest another decision point.

Koester’s database and algorithmic tools were put to heavy use during the Ewasko search. Under Pylman’s guidance, search teams were sent from the location of Ewasko’s car up to the top of Quail Mountain; south to Keys View; deep into Juniper Flats; and out through a number of less likely but nonetheless possible areas, in an exhaustive, step-by-step elimination of the surrounding landscape. Everywhere they went, the question was the same: What would Ewasko do? Would he take the path that arcs gradually southwest, toward the town of Desert Hot Springs, or would he follow a dry wash that slowly fades into the landscape in a distant canyon? Would he have diverted from the trail altogether? Each search team was sent to test a different answer to these questions.

Smith Water Canyon, a search location suggested by data from Ewasko’s phone. Philip Montgomery for The New York Times

By Saturday afternoon, June 26, volunteers were arriving from throughout Southern California, and an incident command post was established near a bulbous natural rock formation known as Cap Rock. There, a 6-by-9-foot map of the area was taped together and layered with each team’s daily GPS tracks and the routes of helicopter flights. Every square inch, it seemed, had been covered. One team stumbled on a red bandanna at the foot of Quail Mountain. Another reportedly saw lights one night on a ridge. A bloodhound was exposed to clothes found in Ewasko’s rental car, then brought on the trail. There were more helicopter flights and more hikes. Despite the impeccable logic of lost-person algorithms and the interpretive allure of Big Data, however, Ewasko could not be found.

There is an unsettling truth often revealed by search-and-rescue operations: Every landscape reveals more of itself as you search it. The mathematician Benoit Mandelbrot once observed that the British coastline can never be fully mapped because the more closely you examine it — not just the bays, but the inlets within the bays, and the streams within the inlets — the longer the coast becomes. Although Joshua Tree comprises more than 1,200 square miles of desert with a clear and bounded border, its interior is a constantly changing landscape of hills, canyons, riverbeds, caves and alcoves large enough to hide a human from view. Solid canyon walls reveal themselves, on closer inspection, to be loose agglomerations of huge rocks, hiding crevasses as large as living rooms. The park is, in a sense, immeasurable. And now Ewasko’s case, like Joshua Tree itself, was becoming fractal: The more ground the search covered, the more there was to see. As Pete Carlson of the Riverside Mountain Rescue Unit put it to me, “If you haven’t found them, then they’re someplace you haven’t looked yet.”

On July 5, 2010, 11 days after Mary Winston got through to park rangers to report Ewasko missing, the official search was called off. Regional resources had been exhausted. Teams broke up or were assigned elsewhere in the state. Ewasko, it was assumed, simply could not have survived that long without food and water, in clothes ill suited for the desert’s extreme temperatures. “After a while,” Carlson said to me, “where else do you look?”

Eight years after he disappeared, Bill Ewasko is still missing. While the official search lasted less than two weeks, unofficially it never ended. A loose group of sleuths with no personal connection to the Ewasko family — backcountry hikers, outdoors enthusiasts, online obsessives — has joined the hunt, refusing to give up on a man they never knew. As it happens, we live in something of a golden age for amateur investigations. Armchair detectives have at their disposal an array of internet resources, like WebSleuths, a forum with more than 140,000 registered users dedicated to examining unsolved crimes, including missing-persons reports. Reddit, too, has become a gathering place for online detectives, with multiple threads about the search for Bill Ewasko. The Ewasko search also continues to attract dozens of commenters to an irregularly updated thread hosted by the Mount San Jacinto Outdoor Recreation forum. There, avid hikers have collectively posted more than 500 times about Ewasko since May 2012. Since the official search for Bill Ewasko was called off, strangers have cataloged more than 1,000 miles of hiking routes, with new attempts continuing to this day. This makes the search for Bill Ewasko one of the most geographically extensive amateur missing-person searches in U.S. history. Ewasko may not be found alive, these searchers believe, but he will be found.

Tracking down the lost, however, is more than just an effort to solve a mystery. The intensity that many of these investigators bring to their work suggests a fundamental discomfort with the very idea of disappearance in the 21st century: People should not be able to disappear, not in this day and age. Not everyone who is lost actually wants to be found. For this reason, the searcher’s compulsion is both a promise and a threat.

When Mike Melson became interested in the Ewasko case, it was nearly two years after Ewasko’s disappearance, in the spring of 2012. A computer scientist by training, Melson knew he possessed technical skills that might shed light on Ewasko’s fate. Informed by more than a decade’s work with law enforcement to track cellphone data, Melson had developed a proprietary forensics program called CellHawk capable of turning raw cellular information into usable search maps. “My philosophy is: The data says what the data says,” he told me.

Ewasko’s car turned up in this parking lot in Juniper Flats. Philip Montgomery for The New York Times

Most cellphones “ping” radio towers on a regular basis, a kind of digital check-in to ensure that they can access the network when needed. That ping also supplies information that can be used to estimate distance, like how far a phone is from a given tower. This data can be formally requested by the police, if, for example, investigators are trying to track a criminal suspect or to locate a missing person. Using cellphone data in collaboration with local law enforcement, Melson has cracked multiple missing-persons cases, including that of two teenage boys who disappeared in North Carolina. Paying closer attention to the exact moment at which the boys’ phones abruptly left the cellular network, Melson arrived at a macabre but accurate conclusion: The boys had driven into water. Acting on Melson’s tip, the police found their bodies in a canal that was 50 miles away from the last tower pinged.

It was not just the prospect of solving a technical challenge that brought Melson into the hunt for Bill Ewasko. In 2005, Melson and his wife, Bridget, read an article about Nita Mayo, an English-born mother of four who had disappeared in the Sierra Nevada. The Melsons immediately drove to Donnell Vista, where Mayo disappeared, to help her family continue the search. Although Mayo remains missing, the case affected Melson so profoundly that he and his wife started a faith-based volunteer search-and-rescue service called Trinity Search and Recovery. Melson brings an unusual combination of religious clarity and technical know-how to his work: part New Testament, part new digital tools. Trinity’s tagline — “Your Father in heaven is not willing that any of these little ones should be lost” — was taken from the Book of Matthew, from a passage known as the Parable of the Lost Sheep.

Melson had been following the story of the Ewasko disappearance off and on, both through word of mouth in the search-and-rescue community and through a blog called Other Hand, written by Tom Mahood. Mahood, a former volunteer with the Riverside Mountain Rescue Unit and a retired civil engineer, demonstrated his considerable outdoor tracking abilities with the case of the so-called Death Valley Germans. After more than a year of grueling legwork, in 2009 Mahood and another searcher found the remains of a German family who disappeared in Death Valley 13 years earlier. Although Mahood participated in the official search for Bill Ewasko, helping to clear the region around Quail Mountain, the case later became something of an obsession. Mahood has since published more than 80 blog posts about Ewasko’s disappearance, featuring several hundred photographs, meticulously logged GPS tracks and numerous Google Earth files all documenting this open-ended quest.

Juniper Flats in Joshua Tree National Park, which comprises more than 1,200 square miles of desert. Philip Montgomery for The New York Times

Included in Mahood’s trove of information were some enigmatic cellphone records. These records reveal that, at 6:50 a.m. on Sunday, June 27, 2010, three days after Ewasko last spoke with Mary Winston, his cellphone communicated with a Verizon tower just outside the park’s northwestern edge, above the town of Yucca Valley. This was the first time Ewasko’s phone had registered with any towers since the morning of his disappearance, suggesting that his phone had been turned off until that moment to conserve battery life — or that he had been trapped somewhere without service. The ping was a welcome clue, one that shaped several new routes during the official search operation, but it also presented a mystery: According to this data, Ewasko’s phone was 10.6 miles away from the tower at the time of registration. This placed him so far beyond the official search area that, when rescuers first learned of the ping in 2010, many simply did not believe the data.

The three-day gap — and the ping’s unexpected location — inspired a series of theories and countertheories that continue to be developed to this day. Perhaps the signal was distorted by early-morning thermal effects as the sun rose, throwing off Ewasko’s real position. Perhaps the rocky landscape of Joshua Tree acted as a fun-house mirror, splintering the signal’s accuracy one jagged boulder at a time. One commenter on the Mount San Jacinto Outdoor Recreation forum even suggested that a passing bird’s wings could have thrown off the signal; others, more conspiracy-minded, suggested that the ping had been deliberately staged to mask the true reasons for Ewasko’s disappearance.

As for why his phone pinged only once that morning, there was one especially frustrating theory. Some hikers speculated that perhaps Ewasko finally reached a high-enough point where he was confident he could get a clear signal. He had spent three nights alone in the wilderness; he would have known his phone had little power left. He would have turned his phone on, hoping for coverage — and he found it. But any joy was short-lived: An incoming rush of voice mail messages and texts would have crashed the battery before Ewasko could place a call.

Armed with the cellphone data, Melson drove to Joshua Tree in person to explore Covington Flats, one of several possible sites where Ewasko’s ping might have originated. In a sense, Melson knew, there were two landscapes he needed to explore: the complicated rocky interior of the park and the invisible electromagnetic landscape of cellphone signals washing over it. His goal was to learn if the ping’s suggested 10.6-mile radius could have been accurate. After performing signal tests throughout Covington Flats, however, Melson found that his numerous attempts to mark a specific distance from the Verizon tower revealed sizable margins of error. According to Melson’s measurements, Ewasko’s phone could have been anywhere from a quarter-mile farther away to very nearly at the base of the tower itself, if you factored in reflections off mountains and rocks. The 10.6 miles turned out to be merely a rough guide — a diffuse zone rather than a hard limit around which any future searches should be organized.

Melson also cautioned me that the original 10.6-mile number cannot, in fact, be verified. This turned out to be correct. A spokesman for the Riverside Sheriff’s Department told me that the original cell data no longer exists. What’s more, the 10.6-mile number apparently came from a single technician. In other words, this hugely influential data point, one that has now come to dominate the search for Bill Ewasko, could, in the end, have been nothing but a clerical error.

“Getting into missing-persons cases was a way for me to stimulate my brain,” Adam Marsland told me. We were hiking into a remote region of the park known as Smith Water Canyon, where Marsland had logged more than 140 miles, often alone, looking for Bill Ewasko. Marsland, now 52, was a pop musician living in the suburbs of Los Angeles. He calls himself a “desert rat” and told me he is used to taking long solo hikes in the Mojave and beyond. “I love being a musician,” he said, “but it isn’t an intellectual puzzle most of the time. Developing this hobby was like I wasn’t a musician for a while: I could be a detective.”

Ewasko’s cellphone communicated with a Verizon cell tower just outside the park’s northwestern edge, above the town of Yucca Valley. Philip Montgomery for The New York Times

For Marsland, discovering the Ewasko case on Tom Mahood’s blog was life-changing. “I was going through a period where I felt pretty shut in and bored and kind of isolated,” Marsland said. “I just went down the rabbit hole with Tom’s website and started developing theories of my own.” Working alone at night in his studio, Marsland found himself poring over other websites dedicated to missing persons, like the widely publicized search for Maura Murray, a college student who disappeared in February 2004 after a car accident in rural New Hampshire. He was drawn to the thrill of seeing clues come together, the tantalizing sensation that a secret story was about to reveal itself. Marsland began to feel a pull that internet research alone could not satisfy, so he decided to head out to Joshua Tree and join the search for Bill Ewasko. His car, a battered 2001 Toyota Echo, showed marks of 20 expeditions into the desert on the trail of a man he never met in person.

To hear Marsland tell it, his inaugural trip to the park, on March 1, 2013, bore the full force of revelation. He purchased hiking gear at a Los Angeles outdoors store, booked himself a room at a nearby hotel in Yucca Valley and set off at 6:30 a.m. He managed to get much farther into the park than he expected. But rather than retreat, he pushed on, walking up the side of Smith Water Canyon. At the top of the ridgeline, he found a curious pit. “It was enclosed by rocks, and you couldn’t really see it from the side,” Marsland told me. “I remember thinking that this is exactly the kind of place where you would expect Bill to be: someplace where he had fallen down, he couldn’t get out and you would never find him. I remember thinking that I had to clear this pit. I had to crawl right up to the edge of it and look down, and I remember being so afraid that I would fall into the pit myself.”

The pit contained no bodies, or even clues, but that moment of possibility was everything. Marsland began drinking less, losing nearly 40 pounds as he reoriented his free time around this quest to find a stranger. “I crossed the line from being somebody who just sat in his room and passively participated in something to being actively involved,” he said. “It was a big moment for me, and it led to a lot of other good things happening in my life.”

In Joshua Tree National Park. Philip Montgomery for The New York Times

Marsland began documenting his hikes for Mahood’s website, posting lengthy and thoughtful reports over the course of more than four years. When I pointed out that he is now one of the most experienced searchers, with detailed knowledge of Joshua Tree’s backcountry, he laughed. “I’m just one guy looking around,” he replied, “and maybe somebody else might even do a better job. I’m just the guy that went.”

By May 2014, the total mileage accumulated in these unofficial excursions by interested outsiders had surpassed the original search-and-rescue operation. Looking for Bill Ewasko had pulled Marsland out of his studio in suburban Los Angeles and into some of the most remote stretches of Joshua Tree National Park. Spurred by this experience of looking for a stranger, Marsland realized that he should perhaps spend more time looking for himself. He made an even bigger leap, selling his possessions not long after our hike together and moving to Southeast Asia, where he plans to drift for a while before deciding if the move should be permanent. “I think all of us need some sense of a far horizon in our lives,” he said.

Mary Winston still cannot bring herself to visit Joshua Tree. In a sense, she said, people like Marsland, Mahood and Dave Pylman are doing it for her, looking for a way to end this story that remains painfully incomplete. “As far as closure, there’s no such thing,” she told me. “Even now, if they find Bill or not, there’s still no closure.”

Mahood has indicated in a blog post that his own search is winding down. “That said,” he added, “if I had any new ideas that seemed worth a damn, I’d be out in Joshua Tree in a second.” Until then, this park on the edge of Los Angeles remains an unexpected zone of disappearance — a vast landscape where some lost hikers are quickly rescued and others simply walk out on their own. Still others are less fortunate. A young Orange County couple went missing in the park in the summer of 2017; despite an intensive search effort at the height of tourist season, their remains went undiscovered for three months. (Tragically, it turned out to be a murder-suicide.) In the spring of 2017, a Pasadena woman disappeared after a visit to her local pharmacy; she was found two days later, wandering and confused in Joshua Tree. The park seems to pull people in and only sometimes lets them go.

The response to a person’s disappearance can be a turn to online sleuthing, to the definitive appeal of Big Data, to the precision of signal-propagation physics or even to the power of prayer; but it can also lead to an embrace of emotional realism, an acceptance that completely vanishing, even in an age of Google Maps and ubiquitous GPS, is still possible. Nonetheless, Winston said, she appreciates the extraordinary efforts of the original search teams and remains grateful for the attention of people like Marsland and Mahood. The most important thing for her is not just the company — not just knowing that people are still searching but that, after all this time, they still care.