The precise purpose of big, mysterious earthen formations in the Brazilian rainforest still stumps scientists, who have carried out careful analysis of the region’s history.

As the Amazon suffers from deforestation, over 450 big geoglyphs have appeared in one state in Brazil. Created by indigenous people over 2,000 years ago, the sites take up some 5,000 square miles. They probably are not the remnants of villages, but may have been places to gather for rituals, according to the University of Exeter.

Researchers wanted to better understand the land-use history of the geoglyph region, where they think ancient people managed forests of trees like bamboo or palm.

“The fact that these sites lay hidden for centuries beneath mature rainforest really challenges the idea that Amazonian forests are ‘pristine ecosystems,’” Jennifer Watling, a postdoctoral research at the University of São Paulo, said in a statement. “We immediately wanted to know whether the region was already forested when the geoglyphs were built, and to what extent people impacted the landscape to build these earthworks.”

Watling and others learned more about these mysterious ancient sites by analyzing soil samples from them, looking at evidence of historical vegetation and fires the sites might have experienced.

While today, the forests are threatened by huge amounts of clearing, the researchers think that thousands of years ago, indigenous people managed the forests in more sustainable ways. The geoglyphs were built when people made temporary clearings.

“Despite the huge number and density of geoglyph sites in the region, we can be certain that Acre’s forests were never cleared as extensively, or for as long, as they have been in recent years,” Watling said.

All of this happened before Europeans arrived on the continent, at a time when indigenous people managed the forest in their own way.

“Our evidence that Amazonian forests have been managed by indigenous peoples long before European Contact should not be cited as justification for the destructive, unsustainable land-use practiced today,” Watling added. “It should instead serve to highlight the ingenuity of past subsistence regimes that did not lead to forest degradation, and the importance of indigenous knowledge for finding more sustainable land-use alternatives.”

The study has been published in the journal PNAS.