Over the last several years, geologists in Brazil and Bolivia have uncovered massive peculiar caves that, they say, were not created by any natural process. One of the geologists, Heinrich Frank, a professor at the Federal University of Rio Grande do Sul, crawled inside one such tunnel in the Brazilian town of Novo Hamburgo. It was about 15 feet long, he told Discover Magazine, and at the end of the tunnel there were claw marks all over the ceiling.

Frank said it was likely a "paleoburrow" dug by an extinct species of giant sloth, and once he found that tunnel, he began finding them in droves, Discover Magazine reports. Near the Brazilian city of Porto Alegre, for example, Frank and his students found another tunnel four feet wide, 65 feet long and lined with claw marks. They estimate the tunnel was once as long as 250 feet.

Elephant-sized ground sloths, or Megatherium, were endemic to South America. Elephant-sized ground sloths, or Megatherium, were endemic to South America. (Photo: Robert Bruce Horsfall/Wikimedia Commons)

Although some of the tunnels have since been filled with sediment, "they remain readily apparent, standing out like dark, round knots in a dirt bank." According to Discover Magazine:

In his home state of Rio Grande do Sul, in the far south of Brazil, Frank has documented at least 1,500 paleoburrows so far. In Santa Catarina, just to the north, he’s found hundreds more and counting. “In these burrows, sometimes you get the feeling that there’s some creature waiting around the next curve – that’s how much it feels like a prehistoric animal den,” he says.

Fossils for three species of extinct sloths have been found in the area, including Catonyx, Glossotherium and Lestodon, which weighed several tons. Some species of armadillos, such as Pampatherium, Holmesina or Propraopus, which are also extinct and were slightly smaller than the sloths, may also have helped build these tunnels, experts say.

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.