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An extinct species of giant ground sloths called Megatherium are efficient tunnel diggers. Found deep in the jungles of Brazil, these tunnels are large enough for scientists to walk through. The ceilings are high enough for tall men to explore unobstructed and without the need to crouch low.

Since these shelters aren't cramped to provide protection against the elements and predators, why did extinct giant sloths build them anyway? Is the Megatherium capable of using digging tools? These are the questions that baffle the researchers today.

Tens of thousands of years ago, extinct giant sloths roamed South America. These mega creatures are thought to be a primary herbivore. However, it is believed that they are omnivores as well, given their size which can scavenge or crush smaller animals. The Megatherium can grow to as high as twice the size of grown men when standing upright, according to Prehistoric Wildlife.

The extinct giant sloths lived during the Holocene period, right after the decline of the dinosaurs. It is believed that Megatherium walked alongside early men and through the dawn of civilization. Unlike their modern cousins, extinct giant sloths have robust bone construction. This feature, together with short but sizeable tail, is attributed to their slow motion.

When the tunnels were first found in 2000, Brazilian scientist Heinrich Frank counted his discovery of staggering 1,500 tunnels already. The extinct giant sloth built some of them that are as long as 2,000 feet long. Megatherium dug other tunnels are more complex, teeming with junctions and branches, according to Popular Mechanics.

What baffles Frank and his team is that, apparently, Megatherium may have worked in groups. They also manifest the ability to plan since some tunnels were built in portion but were continued later. Meaning, the Brazilian tunnels were built by generations of extinct giant sloths, one after the other. Further, these humongous beasts dug up by using bare hands, as claw marks were left imprinted in the walls of the tunnels.