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

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 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.

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.

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.