A few years earlier, and about 1,700 miles to the southeast, another Brazilian geologist happened upon a different, equally peculiar cave. Heinrich Frank, a professor at the Federal University of Rio Grande do Sul, was zipping down the highway on a Friday afternoon when he passed a construction site in the town of Novo Hamburgo. There, in a bank where excavators had eaten away half of a hill, he saw a peculiar hole.

Local geology doesn’t yield such a sight, so Frank went back a few weeks later and crawled inside. It was a single shaft, about 15 feet long; at its end, while on his back, he found what looked like claw marks all over the ceiling. Unable to identify any natural geological explanation for the cave’s existence, he eventually concluded that it was a “paleoburrow,” dug, he believes, by an extinct species of giant ground sloth.

“I didn’t know there was such a thing as paleoburrows,” says Frank. “I’m a geologist, a professor, and I’d never even heard of them.”

Frank believes the biggest burrows — measuring up to five feet in diameter — were dug by ground sloths. He and his colleagues consider as possibilities several genera that once lived in South America and whose fossil remains suggest adaptation for serious digging: Catonyx, Glossotherium and the massive, several-ton Lestodon. Others believe that extinct armadillos such as Pampatherium, Holmesina or Propraopus, though smaller than the sloths, were responsible for even the largest burrows.