Teotihuacán in its heyday was one of the largest cities in the world, supporting over 100,000 people in an 8-square-mile stretch of what is now Central Mexico. Now, a team of archaeologists have used an aerial scanning technology to see how the landscape was modified on a sweeping scale by the people who lived there.
The team was trying to understand how ancient Teotihuacán was laid out and how the modern cityscape was built over it. Constructed between around 100 BCE and 450 CE, Teotihuacán sits about 30 miles northeast of Mexico City, making it part and parcel of the modern cityscape.
The researchers found that the builders of the ancient city did plenty of excavating—even quarrying bedrock for other construction sites in town—and that 65% of today’s urban features are built on the same alignments as Teotihuacán. The team also found that 205 features from the ancient city have been destroyed by mining operations since 2015. Their results were published this week in PLOS One.
“We found that we need to re-define what past urban landscapes looked like and what their long-term legacies are on the modern landscape,” said Nawa Sugiyama, an anthropologist at the University of California Riverside and the recent study’s lead author, in an email to Gizmodo. “People have been extensively modifying the built environment for millennia, and in urban contexts, like the ancient city of Teotihuacán, they were changing courses of rivers, altering the topography, and affecting the agricultural potential for the area.”
“These changes made nearly two millennia ago still affect how we construct our buildings, align our roads, and terrace our crops,” Sugiyama added.
The technology the team used is lidar, a once-rarified and now near-ubiquitous way of conducting non-invasive archaeological surveys. Lidar is short for “light detection and ranging,” which is pretty much what the tech does: It shoots light at a target surface and times how long it takes for the light to bounce back. Based on those intervals, archaeologists are able to see slight changes in elevation in extremely high resolution. Lidar can cut through forest canopies, which is helpful in finding settlements hidden for centuries, as was the case with a large Maya settlement north of Tikal discovered in 2018.
The engineers who designed Teotihuacán also bent the San Juan and San Lorenzo rivers, which cut through the city. The rivers were bent to conform to the city’s astronomical alignment, another example of the dazzling effort and expertise that went into building the metropolis. “Controlling the flow of water was not only a method for incorporating the river’s path to Teotihuacán’s cosmic city layout, it was also a way to demonstrate their dominance over these natural elements, a feat that required the hands of thousands of workers,” Sugiyama said.
Unfortunately, subterranean structures are invisible to lidar—the technology merely detects changes in the ground elevation. So the team doesn’t know which 200-odd structures were destroyed when stretches of the valley were mined in anticipation of an international airport, plans for which have since been discarded. Even still, in May the Mexican government condemned a private building project that damaged and destroyed elements of the historic city.
“Lidar captures humanity and nature’s cumulative impact on our planet in three dimensions, forcing us to consider the consequences of our actions both in the past and present,” said Thomas Garrison, an archaeologist and remote sensing specialist at the University of Texas at Austin who was not affiliated with the study, in an email. “In showing the direct connections between ancient Teotihuacán and the modern settlements surrounding the ruins, this study makes a convincing case for why archaeology is such an important discipline in the 21st century and not simply a colonialist endeavor for appropriating cultural heritage.”
The research team will collaborate with the Mexican government’s culture bureau (the Instituto Nacional de Antropología e Historia) in using the maps as a sort of benchmark for the current status of Teotihuacán’s cultural heritage. The job is really about finding out what all is there and making sure it doesn’t disappear under more human development.