The temple-city of Angkor Wat in central Cambodia is one of the most popular tourist attractions in Asia. At one time, it served as the capital of the Khmer empire that ruled most of southeast Asia between the 9th and 15th centuries. But a recent study reveals that Angkor Wat is just a piece of the Khmer legacy. Laser scans conducted last year show that the area is studded with undiscovered archaeological sites, including a city in the jungle that may be larger than Cambodia’s current capital city Phnom Penh.

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According to Lara Dunston at The Guardian, archaeologist Damian Evans, research fellow at the École française d’Extrême-Orient and leader of the Cambodian Archaeological Lidar Initiative, attached a laser scanning system known as Lidar to helicopter skids. Similar to radar, Lidar—short for light detection and ranging—pelts the terrain with laser beams, collecting data that later makes a high-definition picture of the ground beneath the vegetation below. Because many of the structures built by the Khmer were made of wood and other biodegradable materials, they have disappeared and been covered by the jungle. But Lidar is able to detect mounds of earth, foundations, walls, roads and other permanent structures not visible through the thick vegetation.

Evans’ project, as described in the Journal of Archeological Science, surveyed 734 square miles of terrain over 90 hours. The results revealed entire population centers and temple complexes hidden in the jungle.

“We have entire cities discovered beneath the forest that no one knew were there,” Evans tells Dunston. Though a previous Lidar survey uncovered part of the city in 2012, the 2015 survey reveals Mahendraparvata's true size.

Travis Andrews at The Washington Post reports that one of the biggest surprises for Evans was discovering a medieval city near the temple of Preah Khan of Kompong Svay, about 60 miles east of Angkor.

“We had spent a decade on the ground … looking for a city that we figured must be around somewhere … surrounding this temple,” Evans tells Andrews. “All of a sudden, the city has more or less instantly appeared on the screen in front of us. It had been hiding in plain sight. A city that we figured wasn’t there just appeared.”

Though researchers have not yet visited and examined many of these remote sites, the aerial study is already rewriting history. Peter Sharrock of London University’s School of Oriental and African Studies tells Dunston that the results of the aerial survey show that the complex of Khmer cities, temples and canals makes it the largest-known empire on earth during the 12th century.

Evans says that the survey also changes the story of the mysterious collapse of the Khmer empire. He tells the Associated Press that historians believed the Khmer migrated to cities in the south when Thai armies began invading their northern stronghold. But his survey shows there are no large cities in the south, calling that idea into question.

The sheer complexity of the Khmer empire and its ability to terraform their surroundings might be what impressed Evans the most about the find. The Khmer cleared thousands of acres of forest, diverted river and produced a water system centuries ahead of its time.

“The broad conclusion to draw from this is that we’ve underestimated how much humans have shaped their environments,” Evans says.