Historical accounts from the 18th century attest that the Upper Tapajós Basin was once densely populated with large villages connected by roads. Nevertheless, for many years, the prevailing theory among archaeologists was that pre-Hispanic settlements in the Amazon were clustered mainly around the fertile lands near the floodplains. Large swaths of the Amazon, particularly regions situated at a distance from major waterways, remain largely unexplored by researchers. Now, as Sarah Kaplan reports for the Washington Post, new research in the savannah-like region near Brazil’s border with Bolivia​ shows that ancient human activity in the Amazon was far more robust and wide-ranging than experts previously thought.

By studying satellite imagery, researchers from the UK and Brazil found traces of 81 settlements in the Upper Tapajós Basin​. The aerial surveys revealed the remains of dozens of geoglyphs—mysterious, geometric earthworks that may have been used during ritual ceremonies. Villages have often been found near, or even inside geoglyphs, and when archaeologists explored 24 of the sites uncovered by the satellite images, they unearthed stone tools, ceramic fragments, garbage piles, and terra preta, an enriched soil that has been found in other parts of the Amazon. According to Nicola Davis of the Guardian, the team also discovered evidence of fortifications, sunken roads and platforms where houses once stood.

Describing their discovery in Nature Communications, the researchers write that they were able to date wood charcoal from the sites to between 1410 and 1460 C.E. Peak activity of other settlements on the southern rim of the Amazon have been dated as far back as the mid-13th century, leading the team to conclude that “an 1800 km stretch of southern Amazonia was occupied by earth-building cultures living in fortified villages [circa] C.E. 1250–1500.”

According to the study authors, the team believes that settlements during this period were even more wide-ranging than historical accounts indicated. Using a computer model, researchers estimated that there could be as many as 1,300 geoglyphs across 400,000 square kilometers (154,441 square miles) of the southern Amazon rainforest. Between 500,000 and 1 million people may have lived in the region, the models suggest.

The new findings from the Upper Tapajós Basin indicate that the stretch of settlements along the southern Amazon was home to an array of cultures. Communities in the region shared some practices, like soil enrichment and fortification techniques. But their ceramic styles and architectural traditions were diverse.

“We are so excited to have found such a wealth of evidence,” José Iriarte, professor of archaeology at the University of Exeter and one of the authors of the study, said in a statement. “Most of the Amazon hasn’t been excavated yet, but studies such as ours mean we are gradually piecing together more and more information about the history of the largest rainforest on the planet.”