Lost Cities - FOUND

February 2025

BBC Science Focus

When archaeologist Stéphen Rostain first started doing fieldwork in the Amazon rainforest about 40 years ago, there was very little interest in the region.

- SANDRINE CEURSTEMONT

Archaeologists studying the history of the Americas before European colonisation were largely focused on the remains of pyramids and temples built by the ancient Mayas who lived in Mexico and Central America. It was believed that for the past 2,000 years or more, small, nomadic tribes had lived in the dense forests of the Amazon, but that civilisation, as we know it, had never reached them.

“We were less than 10 archaeologists working in the Amazon,” says Rostain, now director of research at the French National Centre for Scientific Research (CNRS) in Paris. “Everybody said there’s nothing to find.”

For four decades, Rostain carried out excavations of the ground in the Upano Valley, a part of the Amazon rainforest in Ecuador, painstakingly collecting evidence that seemed to prove otherwise. But his breakthrough finally came in 2021, thanks to imagery captured from the air using light detection and ranging (lidar) technology.

Lidar is like radar, but uses lasers instead of radio signals. By firing pulses of laser light, a lidar device can detect how long it takes for the pulses to be reflected back and uses that information to gauge range. Fire enough pulses at something and you can use the range information to build up a 3D picture, mapping anything the pulses are hitting. In Rostain’s case, a lidar device was attached to the belly of an aircraft that would then fly over the Upano Valley, bouncing laser pulses off the ground below. Using the information gathered during the various flights, he was able to produce high-resolution 3D maps of the area that revealed a vast network of lost cities.

“Lidar gives this magic result where you see the original landscape of the first inhabitants,” he describes. “It’s really impacting.”

When Rostain’s paper announcing his findings was published last year in 2024, it was hailed as a major discovery.