Across northern South America, there are hundreds of colossal tunnels large enough for humans to walk through, but they weren’t dug by men. Nor they were formed by any known geological process. But their creators have left evidence all around the walls and ceilings—giant claw marks.

Geologists call these tunnels “paleoburrow,” and they are believed to have been dug by an extinct species of giant ground sloth.

A large paleoburrow in Brazil. Photo credit: Heinrich Frank