After studying the markings, researchers believe they were made by now-extinct giant ground sloths and giant armadillos! These tunnel systems were actually their burrows. The creatures who made them were certified megafauna — they were at least 20 feet long. But that begged the question: how did a slow-moving giant sloth build a tunnel this complex? Photo Credit: Nobu Tamura / Wikimedia Commons | Robert Bruce Horsfall / Wikimedia Commons