This is pretty frickin cool. My guess?
quote:
Deep inside Brazil, there are tunnels large enough for a person to walk through. They are neatly symmetrical—too neat to have been caused by any known geologic process. And they are lined with claw marks.These megatunnels are probably the handiwork of giant ground sloths—humongous “paleoburrows” that no longer walk the Earth. But tens of thousands of years after these megafauna did their digging, those tunnels still dot this part of South America. Discover has a great feature up about it.
Up until the 2000s, little was known or written about this bounty of holes. But since he came upon his first one near Novo Hamburgo, Brazilian scientist Heinrich Frank has found more than 1,500 tunnels. Frank has found burrows that measure hundreds of feet long. Scientists have discovered one with branching tunnels that, when you measure the thing in its entirety, comes in at 2,000 feet long. It had to have been dug by many creatures over generations, not by one or two giant sloths.
The big open question is, Why? The tunnels appear to be much larger than any burrowing animal would need to get away from bad weather or hungry predators. As Frank tells Discover, “There’s no explanation – not predators, not climate, not humidity. I really don’t know.”
Source: Discover
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Some of that stuff is pretty far out there, but some seems a bit plausible. Lot’s of unknowns in Man’s past.