Giant sloths in South America excavated ENORMOUS tunnels during the Pleistocene. These remarkable megaburrows preserve the claw marks from the beasts that engineered them. More details and image credits here https://www.discovermagazine.com/planet-earth/get-lost-in-mega-tunnels-dug-by-south-american-megafauna

“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.”

Who dug these giant tunnels?

Megatherium americanum was up to 10 times the size of living sloths reaching weights of up to 4 tonnes. On its hind legs, it would have stood a full 3.5 m tall. It is the largest known bipedal mammal https://www.nhm.ac.uk/discover/what-was-megatherium.html

This paper from 2015 provides more fascinating context:

“The general characteristics of these two caves enable us to propose that they originated from the activity of digging ground sloths of the South American Megafauna.” http://www.ppegeo.igc.usp.br/index.php/rbp/article/download/10000/9330

“Their dimensions suggest that they were excavated and inhabited not by individual animals, but by groups of sloths. The original polished walls suggest that the caves were used for extended periods, possibly in the order of centuries.”

“Their morphology is so different from other tunnel systems that were excavated by the Megafauna and found in southern Brazil, that we suggest that this morphology of chamber systems is associated with a specific species of ground sloth, whose ID will be very difficult.”

Frank et al. 2015 #GiantSloths

“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...”

What is the largest animal in Earth history that spent a major chunk of time living underground?

“he squeezed through an elliptical shaft roughly 4 feet wide, 65 feet long and lined with claw marks. Extrapolating from the original size of the hill sliced away for the highway, he calculated that the original burrow was about 250 feet long, not counting for twists and turns”