Archaeologists have been trying to uncover the ancient city of Mahendraparvata for 150 years.

The city was one of the first capitals of the Khmer Empire, but it emptied after a new capital was built in Angkor.

For centuries, the site has been covered by dense trees that make it hard to observe.

But researchers recently published a paper definitively locating the city in a Cambodian mountain range.

Its layout is leading researchers to rethink their understanding of how ancient cities were built.

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The city of Mahendraparvata was never lost — just hidden.

For centuries, the ancient city has been buried under a dense canopy in the Cambodian jungle. It was one of the first capitals of the Khmer Empire, which controlled large swaths of Southeast Asia from the ninth to 15th centuries. Over the last 150 years, archaeologists have uncovered artifacts that they suspect came from Mahendraparvata, but they didn't have enough evidence to support the link — until now.

A new paper has confirmed the location of Mahendraparvata in a Cambodian mountain range called Phnom Kulen. From 2012 to 2017, a team of international researchers observed nearly 600 newly discovered remnants of the ancient civilization. These included shrines, a royal palace, and a pyramid-shaped temple.

The city is arranged in a giant grid, which would make Mahendraparvata one of the first engineered landscapes of the era. But until recently, according to the researchers behind the paper, the area has received "strikingly little attention."

"It is almost entirely missing from archaeological maps, except as a scatter of points denoting the remains of some brick temples," they said.

Now that the civilization has a confirmed location, it could give new insight into how cities were built more than 1,000 years ago.

Laser scanners helped reveal the city beneath the trees

Mahendraparvata may not have been a thriving city for long. It served as a capital during the years between the eighth and ninth centuries, but archaeologists believe it may only have functioned that way for decades before the Khmer Empire moved its operations to Angkor.

After the city was abandoned, its wooden structures likely disintegrated, leaving scant evidence that it ever existed. What little was known about Mahendraparvata was preserved in ancient inscriptions.

At the start of the 20th century, archaeologists began to speculate that the city was located on an elongated plateau in the region. The suspicion grew stronger in the 1930s, when historian Philippe Stern found temples and water channels in that area that dated back the ninth century.

But historians still had trouble confirming that the ruins belonged to Mahendraparvata.

Researchers wade through dense vegetation in Phnom Kulen. JB Chevance

The site is covered by dense trees that make it hard to observe the forest floor. It's also somewhat dangerous to visit: From the early 1970s to the late 1990s, the area was occupied by the Khmer Rouge, a brutal regime that carried out genocide in Cambodia, and the terrain is still scattered with land mines.

A research group led by the non-profit Archaeology and Development Foundation found a way around this problem — they used laser scanners from aircrafts to peer through the forest canopy. The lasers sent out pulses of infrared light, which hit the ground then bounced back toward the aircraft, where they were recorded by the scanner. Researchers used the scans to create 3D models of the terrain.

In 2013, they published a paper that came close to identifying Mahendraparvata as an urban settlement in Phnom Kulen. For the next few years, they continued to scan the region and corroborate their findings with on-the-ground excavations. Their latest paper, published on Tuesday, confirms that they have indeed located the hidden city.

'It is totally unique in the Khmer world'

Mahendraparvata appears to have served as a testing ground for innovations and urban layouts that came to define the Khmer Empire for the next 500 years. The city is "distinctly 'urban'," according to the researchers. A network of highways divides it into a grid. Within that grid, the land is parceled into city blocks about a mile long and a mile wide.

The researchers also found evidence of a large-scale water-management system that included dams and an unfinished reservoir.

"Even if it was never functional, the reservoir at Mahendraparvata was a prototype for the vast artificial lakes that would become a defining feature of later Angkor," the team wrote.

Researchers observe the terrain in Phnom Kulen. Nina Hofer

Based on this evidence, the researchers believe the city once hosted a royal court and a "substantial population" of administrative staff.

But it didn't have a crowded city center cordoned off by a wall or moat, nor was it surrounded by sparse neighborhoods. In those ways, Mahendraparvata defies the neat patterns that researchers previously attributed to cities in the region.

"It is totally unique in the Khmer world," the researchers wrote. They added that the layout encourages historians not to think of Khmer cities as "neatly defined, well delineated, and densely inhabited spaces," but to "consider them instead as components of a messy and complex continuum of urban and rural space."

Understanding the city's layout also gives researchers a fuller picture of what the Khmer Empire looked like overall.

Their paper "effectively draws to a close 150 years of archaeological mapping work in the Greater Angkor region," the group wrote. The next step, they said, is to "resolve basic questions" about the people who lived there.