Underground Chambers and Tunnels Confirmed at Mitla

Recent archaeological investigations at Mitla, a significant Zapotec site in Oaxaca, Mexico, have confirmed the existence of a vast network of subterranean chambers and tunnels. Using advanced geophysical technologies, including ground-penetrating radar, electric resistivity tomography, and seismic tomography, the team from ARX Project Lyobaa has provided the first solid evidence of what was long suspected: a hidden underground labyrinth that may have served as a gateway to the Zapotec underworld.

Installing electrode plates for Electric Resistivity Tomography (ERT) scan of the Calvario Pyramid. (© Marco M. Vigato/ Arx Project)

Mitla: A Ceremonial Hub of the Zapotecs

Mitla, located in the central valley of Oaxaca, 40 kilometers (25 miles) southeast of Oaxaca City, was a ceremonial center for the Zapotec civilization. After the fall of Monte Albán around AD 750, Mitla emerged as the new religious and political capital. It reached its architectural zenith between the 8th and 15th centuries AD.

The site is renowned for its monumental architecture, especially its intricate geometric stonework, called Grecas, found on both exterior and interior walls of the buildings.

Five principal groups of structures make up the site: the Church Group, the Columns Group, the Arroyo Group, the Adobe Group, and the South Group. While the architectural style varies, the site is famous for its massive stone constructions and large subterranean tombs, particularly in the Columns Group.

• Searching for the Lost Subterranean Worlds of Mitla, Mexico
• Enormous Ancient Building Identified Beneath Monte Albán’s Main Plaza

Uncovering Subterranean Secrets: Project Lyobaa

Despite numerous excavations over the years, the underground structures at Mitla have remained elusive. Spanish accounts from the 16th and 17th centuries, particularly those of Father Francisco de Burgoa, described an extensive underground labyrinth, considered by the Zapotecs to be an entrance to the underworld. According to these accounts, four chambers beneath Mitla’s temples served as the burial sites for high priests and kings, connected to a cavern that extended for leagues beneath the ground.

In 2022, Project Lyobaa—led by experts from the Mexican National Institute of History and Anthropology (INAH), the National Autonomous University of Mexico (UNAM), and the ARX Association for Archaeological Research and Exploration—began the first modern exploration of Mitla's underground using non-invasive techniques. The team focused on mapping the subterranean landscape to identify voids or anomalies that could represent man-made structures, tunnels, or natural caverns.

Electric Resistivity Tomography (ERT) cross-section of the access ramp to the east of the Church of San Pablo Apostol, showing a set of potential voids (in green) at depths between 1.5 and 12 meters below ground with a linear anomaly suggesting a north-south oriented tunnel. (© Marco M. Vigato/ Arx Project)

The first season’s investigations targeted the Church Group and Columns Group. Ground-penetrating radar revealed a significant void beneath the Church of San Pablo Apostol, extending to a depth of 6 meters (20 feet). This chamber, measuring 15 meters by 10 meters (50 by 33 feet), appeared connected to deeper, yet unexplored cavities. In the Columns Group, geophysical scans identified a complex of potential tunnels and chambers, suggesting an earlier construction phase of the Palace of the Columns, possibly dating back to the 8th or 9th century AD.

• Mitla Mosaics: A Coded Language May Plaster the Walls of a Zapotec City of the Dead
• Investigations Reveal Secrets of Hidden Tunnels Beneath Colonial City in Mexico

Second Season Expands Discovery

In 2023, Project Lyobaa expanded its research to include the Adobe, Arroyo, and South Groups. The findings from the second season confirmed an interconnected network of subterranean spaces beneath all the major architectural groups at Mitla. In the Church Group, the research team used seismic tomography to better define the underground structures, identifying additional tunnels that may connect to the chambers beneath the church.

ERT scan of patio H in the Arroyo Group, showing location of possible tomb. (© Marco M. Vigato/ Arx Project)

Similarly, the Arroyo Group revealed potential burial chambers and a tunnel-like anomaly, extending beneath one of its main patios. These discoveries mirror earlier findings at the Columns Group, where tunnels and chambers are believed to connect several of the site’s most important structures. Under the Adobe Group’s pyramid, seismic scans uncovered two large rectangular chambers, each measuring approximately 3 by 4 meters (10 by 14 feet), connected by angular tunnels. This adds to the growing body of evidence that Mitla’s underground was extensively modified by the Zapotecs.

Implications for Future Research

The confirmation of a vast underground network at Mitla opens up exciting possibilities for understanding the site's role in Zapotec spiritual and ceremonial life. The Zapotec believed that Mitla was the entrance to the underworld, known as Lyobaa, a place of the dead and a conduit between the living and the divine. The newly discovered chambers and tunnels may have been used for religious rituals or as burial crypts for elite members of society, as suggested by the historical accounts of Father Burgoa.

Further exploration of the geophysical anomalies is planned, with researchers considering minimally invasive methods such as drilling small holes to insert micro-cameras into the chambers and tunnels. This approach could provide valuable insights into the layout and condition of the subterranean structures without disturbing the overlying architecture.

If confirmed, these tunnels and chambers could reveal much about the religious practices of the Zapotec and provide insight into how the site functioned during its peak. Additionally, the identification of a potential landslide near the site could explain Mitla's partial abandonment in the late Postclassic period, prior to the Aztec and Spanish conquests.

The Project Lyobaa findings bring us closer to solving one of Mesoamerica's great archaeological mysteries. The vast underground network beneath Mitla, which was long considered a myth, now seems to be real. Future exploration promises to shed light on how this sacred site was used by the Zapotecs and how it fits into their cosmology. These discoveries could further enhance Mitla’s status as one of the most important archaeological sites in Mexico.

Learn more abut how you can get involved and support this hunt for the Zapotec underworld here.

Top image: ERT cross-section scan of the Calvario Pyramid at Mitla, showing solid core and potential hollow areas within the body of the pyramid and under the north-east corner.      Source: © Marco M. Vigato/ Arx Project

By Gary Manners