Who built 2,000-foot ancient tunnels in Brazil and Argentina? Not humans, not nature, but...

Scientists discovered huge tunnels in Brazil and Argentina decades ago and were surprised to see such complex creations made when ancient humans walked the planet. To create such vast tunnels would not have been easy in the absence of complex equipment. Experts think that it is also almost impossible that geological processes shaped these tunnels. The mystery of who built these tunnels was solved in 2018 when a study published in Science Advances revealed that these tunnels were actually made by a creature that has now gone extinct. According to researchers, giant ground sloths burrowed through the ground to make the tunnels. Geology professor Heinrich Frank first discovered these tunnels. These structures are over 1,968 feet long and nearly six feet high. What's more, over 1,500 such tunnels were found in the Rio Grande do Sul region in southern Brazil alone. Humans didn't possess the tools and craft to make them, nor could they have been the work of nature.

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Frank noted that these “paleoburrows” had big claw scratches, set up in a parallel layout. This was a clue that these tunnels were the handiwork of giant creatures. “There’s no geological process in the world that produces long tunnels with a circular or elliptical cross-section, which branch and rise and fall, with claw marks on the walls,” Frank told Discover. He noted that he had come across "dozens of caves that have inorganic origins," and he is sure that "digging animals had no role in their creation."

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To prove that the tunnels weren't made by natural processes, scientists point to the fact that the sheer scale, geometry, and features show that they did not originate from geological processes like erosion or lava tubes. They also do not resemble any known anthropogenic structures. They point to the giant claw marks to state that a prehistoric animal made these tunnels, more specifically giant ground sloth (Megatherium or related genera), This sloth scuttled through the grounds of South America during the Pleistocene epoch, some 8,000–10,000 years ago. They weren't ordinary sloths, but resembled elephants. Their limbs were strong, and their claws were huge, which allowed them to excavate the ground to build the tunnels.

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They were built over several generations, as proven by their length and the area they were spread across. They most likely served as shelters for the sloths, who were herbivores. Human footprints, seemingly showing that they followed the sloths, have also been found in the tunnels. “It is possible that the behaviour was playful, but human interactions with sloths are probably better interpreted in the context of stalking and/or hunting,” according to researchers.