The world's biggest religious complex, some four times the size of Vatican City, is Cambodia's Angkor Wat, built in 1150 but long lost to history. Archaeological work since the 19th century has uncovered pieces of it, but it wasn't until last year that researchers got a detailed sense of how the site once looked—thanks to some high-tech lasers, the BBC reports. In just two weeks, experts were able to map about 140 square miles in the area; they offer a brilliant picture of what the BBC calls "the greatest medieval city on Earth." Its population approached 1 million before its rulers left it behind in the 15th century.
To achieve their findings, archaeologists sent a helicopter over the area. It carried a sensing technology called lidar, which shot lasers over the jungle in four-second intervals, the BBC reports. Lidar can "see" right through the plant growth, China Topix notes. And it revealed tiny changes in the topography of the site, allowing researchers to discover the hidden remains in the region—including what's left of the city of Mahendraparvata, thought to have been built 300 years before Angkor. "You have this kind of sudden eureka moment where you bring the data up on screen the first time and there it is—this ancient city very clearly in front of you," one expert says. The results showed lost temples, roads, and incredible hydraulic systems that allowed citizens to use monsoon water to their benefit. Floods, however, eventually drove people away. Another "lost city" turned up recently in Mexico. (Read more Cambodia stories.)