He survived an avalanche on Everest. Then he disappeared on a California mountain.

Quang Thân had summitted some of the world's tallest peaks: Kilimanjaro, Aconagua, Denali. He had attempted Everest twice and survived an avalanche there in 2005. He was a man at ease in the wilderness, accustomed to extremes.

His extensive experience makes what happened to Thân even more confounding. During a day hike on a much lesser-known California mountain he previously summitted, Thân seemingly vanished.

It was Aug. 21. Thân, 66, was with two other hikers summitting Split Mountain in California's Sierra Nevada, a 400-mile range home to Yosemite National Park, the tallest peak in the contiguous United States (Mount Whitney) and a long section of the Pacific Crest Trail.

At some point just 1,000 feet from the summit, Thân fell a bit behind and waved them on.

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His companions, who Thân met through a Facebook group for Vietnamese hikers, told Thân to stay put as they summitted the 14,064-foot peak. They were able to keep an eye on Thân for all but 20 minutes of their journey to the summit, said Dave Fox, a district ranger at Sequoia National park, but when they returned, Thân was gone.

They began searching in earnest at a nearby lake where Thân had stashed some gear. The gear was there but he wasn't.

Later that day, authorities began a search that eventually included helicopters, infrared, drones and K9 teams. After 10 days of efforts that included more than a dozen agencies, nobody found a single trace of Thân and the active search was called off.

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'Any person can go missing'

So how does a man with so much mountaineering experience vanish on a relatively simple day hike and remain missing despite an exhaustive search?

"It can happen to anybody," said Fox, who was one of the incident commanders in charge of the search effort for Thân.

"We do not put handrails in our mountains," Fox said. "It is wilderness, it can be treacherous. There's no guaranteed level of safety and any person can go missing."

Fox learned that in 1996, when one of his fellow rangers went missing while on a routine patrol. Fox joined in the two-week search for the ranger, a backcountry veteran named Randy Morgenson. Though the search is considered one of the most extensive in national park history, it turned up nothing and had to be called off.

No one knew what happened to Morgenson until five years later, when a trail worker found him in a stream drainage. It's believed he fell to his death as he was attempting to cross the stream.

In 1942, a World War II airman from Ohio named Ernest Glenn went down in a plane in King's Canyon National Park during a training flight. He was found by a hiker 65 years later in 2005 after he had been preserved by a glacier.

Most recently in 2019, hikers found the remains of a Japanese American man named Giichi Matsamura who was being held against his will at the Manzanar internment camp, which lies in the shadow of the Eastern Sierra. Matsamura was caught in a freak snowstorm in 1945 after getting separated from other members of the camp during a hike.

While some people are found years or decades after going missing, others remain lost to the wilderness.

Matthew Greene, a 39-year-old math teacher from Bethlehem, Pennsylvania, went missing in July 2013 on what was supposed to be a long day hike near Mammoth Lakes. He's never been found.

Matthew Kraft, a 24-year-old platoon leader with the 1st Marine Division at Camp Pendleton, disappeared in February 2019 after going on a two-week solo backcountry ski trip along the High Sierra Route. Snowstorms soon covered up any trace of him and he was declared dead.

In Sequoia and Kings national parks alone, there are nine people who have never been found, Fox said.

Thân became the ninth name on that list.

'We should have seen him'

Unlike some who remain missing for years, rescuers knew the exact mountain Thân he had been on, and they knew his hiking partners had only lost sight of him for 20 minutes. That should have made finding him a lot easier.

"We don't even understand why haven't we found him yet," said Cpl. Victor Lawson, one of the incident commanders in charge of the search for Thân.

"We've been throwing everything at this thing," he said. "We should have seen him, something. And we haven't and that's frustrating to us."

While it's one of the easier of California's so-called 14-ers (mountains over 14,000 feet tall) to scale, Thân and his hiking partners were approaching it from a more advanced route on a steep face filled with loose scree, Fox said.

At one point, one of the hikers wanted to turn around but Thân and the third hiker wanted to continue, Fox said.

When Thân later slowed down and waved on the other two hikers, they didn't notice anything wrong with him though altitude sickness and the accompanying confusion could have easily set in during the 90 minutes they were separated, Fox added.

Thân also was a minimalist hiker who didn't carry much food, water or gear, including maps, he said.

Rescuers think it's possible Thân either got altitude sickness or experienced some other medical episode, or simply lost the route, and took a fatal wrong turn. The trail on Split Mountain isn't that well-defined and plenty of other hikers have reported going down the wrong way before correcting their mistake.

If Thân got off trail and then took a tumble, depending on the location, he could have fallen thousands of feet. In the space between are deep crevices, car-sized boulders, and loose scree, all on an extremely steep slope.

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"The crevices out there can be very, very deep," Fox said. "A person could fall into one ... It's dark and deep and rocks continue to fall into them. It's unstable enough where we would not consider sending searchers."

That was among the many factors taken into account when Fox recommended the active search be called off.

"It's probably the hardest decision I've made in my career," Fox said. "The family is never going to get closure if we can't find some part of Quang out there ... It not only haunts the family but it haunts the rescuers, as well."

Thân's family has not returned requests for comment. He lived in Southern California with his wife and was an adoring uncle, according to social media posts by his nieces.

"He is desperately missed by his family and friends," wrote one of his nieces under the name Tiffany AD on Aug. 27. "We know his chances of survival decrease every day and have faith in his experience, tenacity and abilities but worry the situation he's in is not up to him. I hope for the opportunity to scold him about best hiking practices and hear more of his incessant dad jokes."

In a 1987 article published in The Oklahoman, Thân recounted a harrowing solo summit of Argentina's Aconcagua, the highest mountain in the Americas at 22,837 feet.

Within an hour of the summit, Thân and other climbers spotted an incoming storm and everyone but him decided to turn back. He was lucky to be alive after temperatures dropped, winds reached around 50 miles an hour, and snow began blowing sideways and created whiteout conditions.

Thân knew he had barely lived to tell the tale and vowed never to hike alone again. Later he survived an avalanche on Everest.

He told The Oklahoman that the risks were worth it.

"That's the beauty of climbing," he said. "If you know you can do it, what's the point?"

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This article originally appeared on USA TODAY: He survived an avalanche on Everest only to disappear in California