The Amazon rainforest, with its dense vegetation, is really good at hiding stuff. But all the signs of ancient civilizations hiding beneath its lush green canopies are no match for modern archaeologists using LiDAR technology, a.k.a. laser scanning tech that can see through the dense forest canopy.

According to a study published in Nature, researchers used this high-tech scanning system to uncover 396 previously unknown earthworks built by the ancient Aquiry civilization in southwestern Brazil. That adds to the around 1,700 known sites and suggests that more than 20,000 monumental sites might be waiting to be discovered beneath all those trees.

The structures, built between roughly 600 B.C. and A.D. 850, were massive squares, circles, and rectangles that likely served as gathering spots where social, political, and ceremonial activities took place. Around them were longhouses occupied by farming families who grew maize and squash.

Lasers Revealed Hundreds of Ancient Structures Hidden Beneath the Amazon Rainforest

The scale of it all is just more evidence to throw onto the pile that has already, long ago, forever changed our perception of the Amazon as a vast expanse of untouched wilderness before the Europeans arrived. Colonization did not bring civilization. It just brought a different, more oppressive version of it. As the new evidence indicates, the new region was densely populated and may have supported between 1.25 million and 3 million people at its peak.

The most impressive aspect of it, however, is how it managed to stay so hidden for so long, waiting for only the most cutting-edge technology to reveal what was once a sprawling, bustling ancient South American city. This was no small collective of huts. There were roads and embankments and geometric complexes that were invisible even on the ground, including one especially elaborate site that combined nested rectangular enclosures connected by wide avenues.

This was a whole city, and all buried underground.

For as much as this find reveals, the researchers say that the Aquiry are still largely mysterious to us. We don’t know why their civilization rapidly declined between A.D. 850 and 950, but we can take a wild guess since it roughly coincides with the fall of other pre-Columbian societies across the Americas.