For a decade, archaeologists have been racing to study deep mysterious trenches slashed through the soil of the Amazon. Rings, squares and hexagons have pointed to long-forgotten human settlements in a region previously neglected by researchers.

Now, the discovery of 81 new archaeological sites has confirmed that up to a million people lived in an area spanning just 7 percent of the basin before the arrival of Europeans.

Old estimates of up to two million inhabitants across the whole of the Amazon need to be thrown out the window, the researchers wrote in a study published in Nature Communications,