Underneath the mountains and deserts of the U.S. West lie hundreds of thousands of abandoned mines, an underground world that can hold serious danger and unexpected wonder.

They are a legacy of the region's prospecting past, when almost anyone could dig a mine and then walk away, with little cleanup required, when it stopped producing.

In Utah alone, the state is trying to seal more than 10,000 open mines with cinderblocks and metal grates after people have died in rock falls and all-terrain-vehicle crashes and from poisonous air over the past three decades. Just this month in Arizona, a prospector broke his left leg and ankle after plunging to the bottom of an old mine shaft. He spent nearly three days there with no food or water fending off rattlesnakes before a friend heard his cries for help.

Still, not everyone wants to see the mines closed. For years, a dedicated subculture of explorers has been slipping underground to see tunnels lined with sparkling quartz, century-old rail cars and caverns that open in the earth like buried ballrooms.

"Nobody has walked the path you're walking for 100 years," said Jeremy MacLee, who uses old mining documents and high-tech safety equipment to find and explore forgotten holes, mostly in Utah.

He also lends his expertise to searches for missing people. That's how he got to know Bill Powell, who looked for his 18-year-old son, Riley, for months before the teenager and his girlfriend were found dead in a mine shaft the outside the small town of Eureka.

The teens' families formed a close bond with MacLee and other volunteer searchers. Despite his painful memories, Bill Powell decided to see what draws his friend to those dark recesses deep in the desert.

"It's a whole different life. The underground life," said Powell, who has a gravelly voice, close-cropped gray beard and a quick smile.

On a recent day, he and MacLee joined a group of friends in front of a mountainside opening near Eureka, wearing helmets, oxygen meters and strong lights, and a carrying stash of extra batteries. Cool air blasted from the opening, cutting through the desert heat.

The group walked between metal tracks that once carried ore carts, making their way through a tunnel shored up in places with squared-off timbers. After nearly a mile, the railcar tracks suddenly dropped into an abyss as the tunnel opened wide into a huge cavern. A hundred years ago, it would be a bustling scene lit with candles and carbide lights, as miners climbed a scaffolding the size of a seven-story building to drill out lead and silver.

Now, it is silent and pitch-black, illuminated only by the searching headlamp beams.

Bill Powell thought of his son, and the trips they took through the desert when he was a kid. Sometimes they'd come across an old mine shaft and toss a rock down, trying to imagine how far it fell. He doesn't do that anymore, not since his son's body was found in one of those pits.

Though the teenager never got to explore a mine like the one his father was in, Bill Powell thought he'd like seeing it. "He'd probably wish he was with me, hanging out."

But the dangers of abandoned mines weigh on Utah officials' minds. There have been 11 deaths since 1982 and more than 40 injuries, including people who entered mines to explore and others who fell in by accident, according to state data. Some abandoned mines become filled with tainted water, as in the toxic 2015 spill from Colorado's Gold King mine, but most in Utah are dry.

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Legally, entering a mine can be considered trespassing in Utah if it has been closed or there are signs posted outside, but prosecutions are rare. Explorers argue it's no more dangerous than outdoor sports ranging from hiking to skiing, which also claim lives in the West.

But there are hazards specific to mines that can be especially dangerous to the unprepared, from abandoned explosives to the potentially fatal low-oxygen air known to miners as "black damp," reclamation specialist Chris Rohrer said. And while some explorers like MacLee go in prepared, many do not.

"It's just a wide open, Wild West thing," he said. "It's a completely uncontrolled situation."

In Arizona, prospector John Waddell fell to the rocky bottom of a mine shaft after the rigging he used to lower himself broke Oct. 15. He survived by sucking moisture out of his shirt before a friend who he'd told about his plans came to check on him.

There are also cases like Riley Powell and his girlfriend, Brelynne "Breezy" Otteson. Prosecutors say an enraged man killed the teenage couple after they visited his girlfriend despite his warning her not to have male visitors. He dumped their bodies in the mine shaft, where they remained for nearly three months before being discovered in March.

Similar cases have occurred in states like Wyoming, Colorado and California. Investigators also searched old mines in Utah and neighboring Nevada after the high-profile 2009 disappearance of Susan Cox Powell, though the 28-year-old Salt Lake City-area wife and mother was never found.

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"Unfortunately, an abandoned mine is probably a good place to dispose of something like that — a person or something you want to hide forever," said Hollie Brown, spokeswoman for the Utah Division of Oil, Gas and Mining.

For the state, the message is as clear as its skull-and-crossbones signs: Stay out and stay alive. The program has been around more than 30 years, and the division has already sealed some 6,000 abandoned mines.

One of the next projects on its list is a onetime stable near an early-1900s mine that used to house mining mules said to be so accustomed to the dim light underground that they had to be blindfolded when they were brought outside, Rohrer said.

In that case, it plans to seal the opening with a metal gate. At other mines, crews build cinderblock walls, backfill with dirt and rocks, or weld rebar over the openings so bats and other wildlife can still get in and out.

"For 150 years, people have dug holes in the ground and brought wealth out of the ground," Rohrer said. "Unfortunately, after they brought that wealth out of the ground, they left that hole behind."