The Amazon rainforest was home to hundreds of settlements and millions of inhabitants, scientists discovered after excavating 81 ancient settlements there.

A study of the discoveries published Tuesday in Nature Communications said it was satellite imagery of the forest that first revealed evidence of the sites, and that 24 of the locations have so far been checked by researchers on the ground.

Carbon dating of objects found at the sites puts them between 1250 and 1500, before Europeans arrived in nations like Brazil and Columbia. The study debunks long-held beliefs that settlers of the rainforest were concentrated on fertile floodplains at the edges of the forest, but that the interior was largely uninhabited.

“The idea that the Amazon was a pristine forest, untouched by humans, home to scattered nomadic populations ... we already knew that was not true,” lead study author and University of Exeter professor Jonas Gregorio de Souza said, The Guardian reported. “The big debate is how populations were distributed in pre-Columbian times in the Amazon.”

Evidence of roads, gardens, ditches, canals, causeways, fish weirs, water reservoirs, and raised fields showed the settlements had complex and developed societies even in seasonally flooded areas, the researchers reported in Nature Communications.

The objects found had enough differences for the researchers to conclude there were a number of different cultures within the settlements, which varied from about 150 yards in diameter to more than 19 hectares (about 47 acres).

De Souza and his colleagues predict there are hundreds more sites yet undiscovered, and that up to a million people may have lived there, in the rainforest interior, The Washington Post reported.