Geologists call these tunnels “paleoburrow,” and they are believed to have been dug by an extinct species of giant ground sloth.
The term was coined by Heinrich Frank, a professor at the Federal University of Rio Grande do Sul, in Brazil, who chanced upon one at a construction site in the town of Novo Hamburgo, in the early 2000s. Up until then, little was known or written about these tunnels in scientific literature. But since he came upon his first, Heinrich Frank and other researchers have discovered more than 1,500 tunnels, in the state of Rio Grande do Sul alone. Some of these burrows measure hundreds of feet long and branched off into several direction.
The largest one measured 2,000 feet long, six feet tall and three to five feet wide. An estimated 4,000 metric tons of dirt and rock were dug out of the hillside to create the burrow. It was evidently the work of not one or two individuals but several generations.
Frank believes the burrows were dug by a genus of giant ground sloths, as large as modern elephants, that once lived in South America until about 10,000 years ago. They were some of the biggest land mammals on earth exceeded in size only by the mammoth. Others believe that extinct armadillos, which were smaller than the giant sloths, were responsible for the burrows.