A city lost in Cambodia’s northwestern jungles for over a millennium has been uncovered by a group of archaeologists from the University of Sydney. Bigger than anyone ever imagined, the find is prompting scientists to revise their previous beliefs about the character — and the eventual collapse — of the Khmer Empire, which ruled this part of Southeast Asia from about 800 to 1400 AD.

Using lasers mounted on a helicopter — a technology called LIDAR — the team was able to penetrate the thick forest vegetation to light up the remains of Mahendraparvata, an ancient Khmer city some 1,200 years old. Archaeologists in Cambodia have long suspected that Mahendraparvata lay buried beneath dense jungle atop the holy mountain of Phnom Kulen (Mountain of Lychees), located about 25 miles (40km) from the world’s largest religious site, the temple complex of Angkor Wat. The ancient cities are said to be connected by a series of roads, dykes, and tunnels.

LIDAR was invented in the 1960s, yet its application in mass archaeological excavations is more recent. It’s especially useful for mapping dense jungle terrain. Last year it was used to find another lost city in the jungles of Honduras. LIDAR works by firing laser pulses from an aircraft to the ground and measuring the distance to create a detailed, three-dimensional map of the area.

“So instead of this kind of very long gradual process, 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,” University of Sydney archaeologist Damian Evans, the study’s lead author, told Australia’s The Age in a video interview from Cambodia.

The Age sent a cameraman to create the 10-minute film above.