The Pyramid of the Sun at Teotihuacan
The Pyramid of the Sun at Teotihuacan is outlined in the likeness of Cerro de Patlachique. Ancient engineers constructed the monument echoing the character of the sacred mountain, just outside the modern metropolis of Mexico City. Credit: Milthon Carlos

For Primo Espinoza, living in the Valley of Teotihuacan 50 years ago was a completely different experience than it is today. What he remembers to be a neighborhood with few houses is now a complex urban system that surrounds the mighty archaeological ruins of the ancient civilization only 40 kilometers from the modern metropolis of Mexico City.

Being a third-generation inhabitant of San Juan Teotihuacan made Espinoza an expert on the area he calls home. He started his career selling mud and obsidian handicrafts to tourists but ended up working as a digger in the archaeological zone, which is a quick 15-minute walk from his house.

But what Espinoza didn’t know until just months ago is that the constructions built 2,000 years ago determined the exact orientation of the street on which he lives: 15° east of north, the same orientation as the massive monuments of Teotihuacan itself.

“Soil is not the same when there’s a city under it; vegetation changes and makes it easier or not to build over….This is how actions from the past affect our decisions of the present.”

That orientation is why every time Espinoza goes out, Cerro Gordo, the mountain that looms over Teotihuacan’s Pyramid of the Moon, dominates the skyline at the end of his street. The alignment is intentional, corresponding to strict urban planning organized during Mesoamerica’s Classic period.

A recent lidar mapping study found that Espinoza’s neighborhood is not unique—around 65% of all modern construction around Teotihuacan (including land divisions, paved and unpaved roads, boundaries, and permanent structures) are aligned with the ancient structures.

The lead author of the study, anthropologist Nawa Sugiyama of the University of California, Riverside, explained how thousand-year-old underground sediments made people unconsciously follow the same construction patterns through time. “Soil is not the same when there’s a city under it,” she said. “Vegetation changes and makes it easier or not to build over.…This is how actions from the past affect our decisions of the present.”

A Developing Threat to Cultural Heritage

Lidar technology has been used for years to find hidden ruins of ancient civilizations in Mexico and around the world, but the new research, published in PLoS ONE, focused on understanding the impact of human activities on the Valley of Teotihuacan’s landscape across time.

People began modifying the landscape more than a millennium ago: Teotihuacan’s engineers quarried hundreds of thousands of kilograms of soil and rock from the valley to construct the city, which grew to a population of about 125,000 at its height around 300 CE. At the same time, they modified the courses of the San Lorenzo and San Juan Rivers to align them for symbolic and calendric reasons.

In addition to examining the urban planning of ancient Teotihuacanos, researchers were also able to analyze the impact of mining and urbanization in the valley over the past century. For instance, they identified more than 200 early features that have been destroyed since the 1960s.

Tezontle and basalt mines, many of them illegal, dot around 150 hectares in the Valley of Teotihuacan, largely driven by demand for construction material in Mexico City.

Ariel Texis, one of the Mexican archaeologists in charge of verifying the team’s lidar findings, got a surprise when he compared the first map to what the hills look like now. “We had [the hills] in the map, but they no longer exist,” he said, having been replaced by open-air mines.

Much of the mining documented by the lidar study was happening at the same time that a new airport serving Mexico City was being built nearby. Although that project was ultimately canceled, another, only about 10 kilometers from Teotihuacan, is currently under construction.

“It’s chaotic [on the periphery of the monumental area]—there are constructions everywhere, it’s sad.”

For Citlali Rosas, archaeologist-in-chief of the Department of Legal and Technical Protection of the Archaeological Zone of Teotihuacan administered by the National Institute of Anthropology and History (INAH), such construction around Teotihuacan is “worrying.” An airport at that distance, she said, will encourage construction of restaurants, hotels, and other businesses catering to tourists that may put delicate artifacts at risk.

A 1988 presidential decree cracked down on illegal extractive activities in the area, but more could be done, said Rosas. On average, the Department of Legal and Technical Protection suspends around 100 construction projects being carried out without INAH’s permission every year.

“It’s chaotic [on the periphery of the monumental area]; there are constructions everywhere, it’s sad,” said Veronica Ortega, an archaeologist with Mexico City’s National School of Anthropology and History who has spent the past 20 years studying ancient structures around the zone. Ortega was not involved in the new study.

Ortega explained that 60% of the territory of the Valley of Teotihuacan has archaeological remains underneath, but much of the area remains unmapped. Lidar efforts like Sugiyama’s would help archaeologists generate a new protection polygon for sediments, artifacts, and remains that lie beyond the archaeological zone.

However, having scientific evidence is not enough, Ortega warned. Stopping the “destruction of one of most important cultural heritages on the planet” will need broad participation and articulation from Mexico’s federal government, municipal authorities, and local communities, she said.

—Humberto Basilio (@HumbertoBasilio), Science Writer

Citation: Basilio, H. (2022), Mapping Teotihuacan’s past, present, and future, Eos, 103, https://doi.org/10.1029/2022EO220018. Published on 6 January 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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