EXETER, ENGLAND—According to a report in New Scientist, areas of Brazil’s Amazon Basin thought to have been made up of virgin forests may have once been densely populated. A retired financial manager in São Paulo contacted archaeologists after he spotted circular earthworks in online satellite images of the southern rim of the Amazon in the Brazilian state of Mato Grosso. More than 80 possible pre-Columbian sites, including small ditched enclosures and large settlements with mounds, plazas, and causeways, dating to between A.D. 1250 and 1500, were detected by scientists during a survey. Excavation at some of the sites revealed pottery and dark earth, which suggests the land had been farmed intensively. Jonas Gregorio de Souza of the University of Exeter said many of the earthworks were likely to have been fortified settlements. The one million people thought to have lived in these settlements likely died of disease and violence brought by European explorers and slavers. To read about other recent discoveries made using satellite imagery, go to “Satellites on the Silk Road.”