Utilizing cutting-edge laser technology, archaeologists have found evidence that densely populated cities once surrounded Angkor Wat and other temple complexes deep in the Cambodian jungle. The discovery of these previously undocumented cities—some so vast that they rivaled the size of the present-day Cambodian capital of Phnom Penh—could rewrite history books and challenge commonly held assumptions about the medieval Khmer Empire.

While one million tourists a year visit Cambodia’s Angkor Wat and gaze in awe of the 900-year-old wonder, something just as remarkable lies hidden beneath their feet. According to an article published yesterday in the Journal of Archaeological Science, researchers have discovered evidence of vast cities and extensive agricultural networks in the ground surrounding Angkor Wat and other medieval temple complexes.

In order to learn the secrets concealed in the floor of the dense jungle, archaeologists took to the air. Utilizing a laser scanner mounted to a helicopter skid pad, researchers with the Cambodian Archaeological Lidar Initiative (CALI), a joint project of the French Institute of Asian Studies and the Cambodian government, spent 90 hours airborne in March and April 2015 gathering detailed topographical data. They scanned 734 square miles of jungle surrounding Angkor Wat and nearby temple complexes with lidar (short for “light detection and ranging”) technology—similar to radar but using pulses of light instead of radio waves—to “see through” the dense jungle vegetation to the ground. Firing 600,000 laser pulses per second, the researchers were able to detect disturbances to the natural environment and create a three-dimensional map of the surface.

After analyzing the data, archaeologists discovered previously undocumented cities surrounding multiple temple complexes built by the Khmer Empire between 900 and 1,400 years ago. “We have entire cities discovered beneath the forest that no one knew were there,” Australian archaeologist Damian Evans, CALI’s principal investigator and technical coordinator, told the Guardian newspaper. One of those settlements was at Preah Khan of Kompong Svay, 60 miles east of Angkor. Evans told the Washington Post that archaeologists had spent a decade on the ground trying to find the city surrounding the temple without any luck. That all changed when researchers studied the lidar data. “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.”

The lidar survey also revealed that these medieval cities were much more technologically sophisticated and densely populated than previously thought. At its peak in the 12th century, Angkor sprawled across more than 350 square miles, a size that London did not reach until seven centuries later. Evans told the Guardian that the new data show that Mahendraparvata, a 1,200-year-old temple city 25 miles west of Angkor, was “the size of Phnom Penh,” a city of 1.5 million people.

Although the perishable materials such as earth, wood and thatch that were used to construct the empire’s buildings—even the royal palaces of Angkorian kings—have long disappeared, the lidar survey detected the remnants of sophisticated water systems, sandstone quarries and royal roads linking temple complexes. Researchers found sculpted mounds of earth on which neighborhoods were built to remain above the floodwaters that arrived in the wet season along with mysterious geometric patterns that could have been gardens. Evans wrote in the Journal of Archaeological Science that Khmer societies re-engineered the jungle landscape “on a scale perhaps unparalleled in the pre-industrial world.” That included a complex system of irrigation canals and dams to divert waterways that could have been built as early as the 5th century, four centuries before the time when archaeologists had believed the water management technology was first developed.

The new findings could mean that the Khmer Empire was once the largest on the planet. “This urban and rural landscape, linked by road and canal networks, now seems to have constituted the largest empire on earth in the 12th century,” Peter Sharrock of London University’s School of Oriental and African Studies told the Guardian.

The lidar survey results may also force historians to reexamine commonly held beliefs about the demise of the Khmer Empire, which was thought to have occurred when Thai invaders sacked Angkor Wat in the 15th century and forced its inhabitants to flee to the south. The research team, however, found no evidence of any densely populated cities south of Angkor or of a sudden mass migration of hundreds of thousands of people. “It calls into question the whole notion of an Angkorian collapse,” Evans told the Guardian.

Evans reports that the information collected by the archaeological team is so bountiful that analysis and field work will continue for years to come. In the meantime, historians will digest the findings and reassess scholarship on the Khmer Empire. “I think that these airborne laser discoveries mark the greatest advance in the past 50 or even 100 years of our knowledge of Angkorian civilization,” Yale University professor emeritus of archaeology and Khmer Empire expert Michael Coe told the Guardian.