The Amazon just got a lot more crowded, at least on the archaeological map.

Researchers flying a plane with remote sensing instruments over about 4,497 square kilometers in Brazil found 396 previously unknown ancient earthworks near the border with Bolivia and Peru. Based on those findings, they concluded the southwest Amazon could hold between 24,000 and 30,000 earthworks in total.

Many of the newly detected structures were geoglyphs, geometric designs built into the ground that can stretch several football fields across. The study links them to the Aquiry civilization, which lived in the Amazon between 600 B.C.E. and C.E. 850.

Archaeologists had found geoglyphs in the region before, but not at this scale.

“We were surprised that we found about 10 times more of the geoglyphs than we ever thought,” said Martti Pärssinen, lead author of the study and an emeritus professor at the University of Helsinki.

The results were published Wednesday in Nature.

Using the number and spread of the newly found geoglyphs, Pärssinen and his team estimate the Aquiry population may have reached as many as three million people at its peak, around C.E. 100 to 300. The study also suggests the civilization may have covered 183,000 square kilometers.

The authors wrote that, if confirmed, the findings could push archaeologists to rethink the size of the Amazon’s population before Spanish and Portuguese colonization.

“What we have studied represents less than 3 percent of the whole Amazonia,” Pärssinen said, adding that it is “probable” that there are other sites where the population density may be similarly high, indicating a precolonial population in the Amazon of possibly tens of millions of people.

“That’s something amazing and something new,” he said.

Heiko Prümers, a researcher at the German Archaeological Institute who was not involved in the study, called the work “really fascinating.”

He said the discovery helps correct the idea “that the Amazon region is a region of ‘untouched nature.’”

But Prümers also said the study has limitations. He said it is unclear whether structures in the same area were counted as separate “sites” or grouped together, which could have affected the total estimates. He also said the high number of sites could reflect “frequent relocations” by the Aquiry rather than high population density.

Researchers think geoglyphs may have been used for ceremonial or cultural purposes, including as gathering places for festivities, but their exact role remains debated.

Pärssinen said archaeological evidence suggests ancient people in the Amazon lived in balance with the environment while still shaping the land with possibly tens of thousands of geoglyphs.

“We could learn something about how they managed the forest in the past,” he said.

Read more from Scientific American.