Around the turn of the century, a number of huge tunnels were discovered in South America. After investigation, the scientists who discovered them found that they are not made by any humans, nor geological process.

Professor of geology, Heinrich Frank, spotted a strange hole embedded in a hill at a construction site as he passed it on the highway, according to Discover. Frank returned to the tunnel, which had been uncovered during excavation, and crawled inside. 

The tunnel was 4.5 meters (15 feet) long. Frank could tell that the tunnel was not made by geological forces, but nothing beyond that. At the end of the tunnel was a much bigger clue, that you probably still don't want to find in a tunnel you've just voluntarily crawled into: a collection of giant claw marks on the ceiling.

“There’s no geological process in the world that produces long tunnels with a circular or elliptical cross-section, which branch and rise and fall, with claw marks on the walls,” Frank told Discover, adding he's "seen dozens of caves that have inorganic origins, and in these cases, it’s very clear that digging animals had no role in their creation.”

The tunnel, along with many others that he and others discovered in Brazil and Argentina, are thought to be made by extinct megafauna. In Frank's case it was likely giant sloths that made the tunnels, 8-10,000 years ago. These creatures are not like the sloths of today, with the primary difference being they were around the size of an African elephant.

In the Rio Grande do Sul area, Frank and his team found over 1,500 tunnels made by the beasts, with the longest stretching for 609 meters (2,000 feet) and standing at 1.8 meters (6 feet) tall. It was likely carved out by teams of sloths over several generations.

Despite their size, there is evidence that humans may have hunted giant sloths. Two hundred fossilized footprints of sloths and humans found in Utah were analyzed by a team in a 2018 study, finding them to be evidence that humans "actively stalked and/or harassed sloths, if not hunted them."

“It is possible that the behavior was playful, but human interactions with sloths are probably better interpreted in the context of stalking and/or hunting,” the palaeontologists wrote. “Sloths would have been formidable prey. Their strong arms and sharp claws gave them a lethal reach and clear advantage in close-quarter encounters.”