Rather than simply multiplying earthworks by a fixed population estimate, the researchers combined several independent lines of evidence. After accounting for earthworks that LiDAR may have missed beneath dense forest, they conclude the total could have been as high as 3.8 million. Dense vegetation also continues to obscure portions of the landscape, meaning many earthworks almost certainly remain undiscovered even after LiDAR surveys. “These ancient achievements constitute an important legacy that is still observable in the soil, and the composition and biodiversity of the contemporary tropical forest,” researchers write. The recent study, “Over 20,000 precolonial earthworks in the Southwest Amazonia,” by Martti Pärssinen et al., appeared in Nature.