Geologists discover claw-marked tunnels dug by a massive ancient sloth or armadillo

Geologists have discovered hundreds of networks of mysterious caves in Brazil that they believe were dug by an enormous prehistoric animal of some kind, such as a giant sloth or giant armadillo, Discover reports. "I'd never seen anything like it before," said Amilcar Adamy, who first stumbled upon a burrow in 2010.

The caves were clearly not formed by any natural geological process — and besides, the walls are covered in gigantic claw marks. Another geologist, Heinrich Frank, separately discovered "paleoburrows," including one that was four feet wide and an estimated 250 feet long. Once he began looking for the tunnels, they turned up everywhere. "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," said Frank. He's found more than 1,500 just in the Brazilian state of Rio Grande do Sul.

Frank added: "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. I've [also] 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."

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The mystery deepened in 2015, when Adamy found a paleoburrow with branches totaling 2,000 feet in length and shafts that were originally six feet tall by three to five feet wide. "This wasn't made by one or two individuals," said Adamy. "It was made by many, over generations." Now there are other known burrows that are estimated to be 3,000 feet in length.

But why so big? Modern armadillos in Brazil are only 65 to 90 pounds and burrow 16-inch diameter holes that are 20 feet long. "What would dig one five feet wide and 250 feet long?" Frank wondered. "There's no explanation — not predators, not climate, not humidity. I really don't know."

And then there is the fact that there were North American giant sloths and giant armadillos, but there are no known paleoburrows in the United States. But that, too, could just be a matter of time. As Greg McDonald, a Bureau of Land Management paleontologist, told Discover: "The fact that we don't have them here could simply be that we've overlooked them."