Archaeologists have discovered the remnants of a massive Maya settlement that was hidden for centuries beneath the lush jungle canopy over northern Guatemala. The findings have caused experts to revise their understanding of the ancient civilization.

Researchers from a local nonprofit called the PACUNAM Foundation have discovered more than 60,000 previously undocumented dwellings, temples, raised highways, and other man-made structures deep in the five-million-acre Maya Biosphere Reserve, a protected area in the Petén region of Guatemala, National Geographic reports.

Using a laser technology called LiDAR (Light Detection and Ranging), the scientists digitally peeled back the lush canopy from aerial images of the jungle to uncover the full extent of this abandoned pre-Columbian civilization.

Based on the biggest set of LiDAR data ever recorded, which covered a region spanning 800 square miles, the astonishing findings indicate that the Maya were much more advanced and interconnected than experts—whose ground research had only revealed widely dispersed and sparsely populated cities—had previously thought.

Their findings are truly groundbreaking. Rather than a site accommodating an estimated population of around 5 million, this new data supports a much denser populous that is closer to 15 million, at a stage of advanced civilization similar to ancient Greece and China.

“The LiDAR images make it clear that this entire region was a settlement system whose scale and population density had been grossly underestimated,” Thomas Garrison, an archaeological expert in digital technology, tells National Geographic.

The unprecedented scale of the discovery showed that people living in Central America 1,200 years ago were much more sophisticated than was previously assumed. Hundreds of formerly unknown structures have been uncovered, including elevated causeways connecting different cities, and developed irrigation systems featuring canals, dikes, and reservoirs, which were capable of supporting a complex agricultural system.

And, apparently, these Maya cities were militarized. The archaeologists were surprised to discover ample evidence of defensive walls, ramparts, and fortresses. “Warfare wasn’t only happening toward the end of the civilization,” explains Garrison. “It was large-scale and systematic, and it endured over many years.”

Unfortunately, while the site is brand new to archaeologists, it has also already been heavily looted. Thousands of pits were found to have been dug by modern-day looters, so any obvious treasures may have long since been pilfered.

But considering that the full area of the vast site—which researchers plan to investigate over three years—spans over 5,000 square miles, who knows what ancient treasures and mysteries have yet to be uncovered?