Vast settlements dating back to the Middle Ages and featuring earthen buildings and pyramids as tall as eight-story buildings have been discovered hidden in the Bolivian Amazon, scientists said Wednesday in a paper published in the journal Nature.

The landmark findings, made possible with the help of advanced laser-mapping technology capable of penetrating the dense vegetation that has long stymied research efforts in the region, upend long-held beliefs among many experts that the region lacked sophisticated societies until European colonizers arrived in the 16th century.