A sinkhole in South Dakota is packed with mammoth fossils that experts have been digging up for half a century. Take a look.

• Dozens of mammoths were trapped in a South Dakota sinkhole over 100,000 years ago .
• A bulldozer uncovered the first fossil 50 years ago, and experts have been finding bones ever since.
• The Mammoth Site museum allows visitors to watch the excavation of these ancient mammals in real-time.

Amid the evergreen forests and picturesque hilltops in the Black Hills of South Dakota is a massive sinkhole time machine.

Tens of thousands of years ago, dozens of mammoths met their doom in this sinkhole death trap deep enough to fit a four-story building.

Today, the sinkhole is a treasure trove for paleontologists who get a rare glimpse into our nation's ancient past.

You can watch these experts uncover its fossilized secrets — from toe to tusk — in real-time at The Mammoth Site museum, which recently celebrated its 50th anniversary.

Over the last half a century, excavators have uncovered fossils from 61 mammoths and many other ancient creatures, and they aren't even halfway through digging to the sinkhole's bottom.

There may still be dozens of undiscovered mammoths in its unexcavated parts.

"I never fail to be inspired when I walk into the museum," Chris Jass, the museum's director of research, told Business Insider. "You're standing right where those animals lived, where they died ."

Take a peek into the Pleistocene past when mammoths roamed over 100,000 years ago.

The first fossil was found 50 years ago.

In the midst of a construction project in 1974, a bulldozer operator realized he'd uncovered a giant tusk .

By the late 1970s, paleontologist Larry Agenbroad insisted a building be constructed over the site to protect it and give researchers time to excavate the sinkhole's huge number of fossils.

He spent decades uncovering mammoths until his death in 2014.

Visitors get an up-close look at how the mammoths died.

One of the things that makes the Mammoth Site special is that visitors can see the fossils as they're uncovered, with paleontologists doing the slow work of excavating them, one bone at a time.

It's a different experience seeing animals lying where they died instead of upright and on display. "It evokes a fairly powerful emotional response in a lot of people," Jass said.

Stuck in a sinkhole would have been an awful way to die, and the bones illustrate that. "They tell the story better than any of us could," he said.

The sinkhole was deadly because its walls were too steep and slippery for mammoths to climb out of.

The Black Hills region of South Dakota is prone to sinkholes, Jass said. Basically, water underground erodes its surroundings forming subterranean caves.

"Eventually those caves grow to sizes that can't support the weight above, and you get a collapse," he said.

The Mammoth Site's sinkhole is 150 by 120 feet and at least 65 feet deep.

The walls were made of a rock called Spearfish shale, which was incredibly slippery. The unlucky mammoths who investigated the sinkhole then had to contend with both the wet rock and their own 10-ton body weight.

Jass compared it to trying to drive over a wet surface when the vehicle's back end loses traction. "You are slipping and sliding as you try to go up very steep slopes," he said.

The mammoths are far older than experts originally thought.

There are two types of mammoths in the sinkhole: the woolly mammoth and its less furry, larger cousin, the Columbian mammoth.

South Dakota is a bit further south than the woolly mammoth's range, which is why most of the site's mammoths are the Columbian species.

For decades, researchers working on the site thought the mammoths were around 26,000 years old.

Now, with revised dating, they've learned the sediments are between 140,000 and 190,000 years old, Jass said.

The new dates have led Jass and his colleagues to evaluate the fossils in a whole new context.

The sinkhole is full of young, male mammoths.

The sinkhole has helped researchers learn more about mammoth behavior, specifically young males' behavior.

Modern elephants travel in matriarchal herds, mostly made up of females and babies. Mammoths were the same.

"When young males reach sexual maturity, they are essentially kicked out of those herds and left to fend for themselves or form groups with other male mammoths," Jass said.

That's likely the reason all the skeletons in the sinkhole appear to be males, mostly between the ages of 12 and 28.

"We think that it was those more impulsive, slightly more adventurous teenagers that got themselves stuck in the sinkhole," he said.

It also shows this wasn't a single catastrophic event that trapped all these mammoths. If it had been, there would have been both males and females and a larger range of ages, Jass said.

Many of the enormous mammoth skeletons are incomplete.

Some of the mammoth skeletons have nicknames, like the nearly intact Napoleon Bonaparte . However, many of the skeletons aren't as complete as Napoleon and require restoration in the museum's basement labs.

For a decomposing body to stay intact, it needs to be buried quickly. Skulls, for example, can get separated from bodies after death fairly easily because there's not a lot keeping them attached, Jass said.

In fact, the museum dubbed a headless fossil Marie Antoinette but switched to Murray when experts realized the mammoth was male.

Tusks also get dislodged once the soft tissue disappears. And the dying mammoths probably played a role, kicking their predecessors and dislodging bones as they tried to get out of the hole, Jass said.

The mammoths weren't alone in getting trapped in the sinkhole.

Excavators have discovered bits and pieces of coyotes, prairie dogs, llamas, extinct camels , and even the near-complete skeleton of a short-faced bear.

Short-faced bears were enormous animals that stood 11 feet tall. Its remains are the most complete besides the mammoths, Jass said.

But there's a reason mammoths make up most of the sinkhole's population.

"Some of those smaller animals were just a bit more agile," Jass said. Even the bear is a surprise, he said. Jess said he would have expected it to manage to crawl out, unless it was injured.

It's also possible floods washed bones into the hole, and some of these smaller animals weren't trapped at all. "There's more for us to uncover, and we've got tantalizing bits of some of those other animals," he said.

There are still dozens more fossils to find.

Dozens of mammoths likely remain undiscovered in the sinkhole. Having the building located around the site allows excavators to take their time finding them because they're protected from any weather that might otherwise degrade them quickly.

Still, there are some areas experts are reluctant to touch. Jass said there's about 45 feet of sinkhole below Napoleon's fossil, but it's such an amazing specimen he doesn't want to move it. Instead, researchers are focusing on a couple of other spots that haven't been explored yet.

Jass estimated the sinkhole contains around 100 mammoths in total. "Those probably won't all be dug up in my lifetime, but eventually, yeah, I would estimate that we'll get to that number," he said.

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