Hidden Under The Rain Forest

National Geographic reports that what we once thought of as untouched, pristine Amazon rainforest was once home to teeming cities and monumental architecture. Once they were abandoned, that's when the rain forest took over.

Satellite images of the upper Amazon Basin taken since 1999 have revealed more than 200 geometric earthworks spanning a distance greater than 155 miles (250 kilometers).

Now researchers estimate that nearly ten times as many such structures—of unknown purpose—may exist undetected under the Amazon's forest cover.