Project Lyobaa - 2023 Season

Cover - Project Lyobaa 2nd Season - v2.jpg

Project Lyobaa - Results from the 2023 Season

by Marco M. Vigato

July 23, 2024

An international research team formed of experts from INAH, UNAM, and the ARX Project has recently released the results from the second season of geophysical research at the archaeological site of Mitla, Oaxaca.

With the use of ground penetrating radar, electric, and seismic tomography, Project Lyobaa confirmed the existence of a network of previously undiscovered subterranean chambers and passageways under the archaeological site of Mitla, Oaxaca.
Mitla was a major ceremonial center of the Zapotec civilization and had its greatest flourishing period between the 8th and 15th centuries CE.
The site is famous for its monumental stone constructions, elaborate subterranean tombs, and intricate geometric mosaic fretwork ornamentation.
The first research season in 2022 focused on the Church Group and the Columns Group of the ancient site, revealing a large void underneath the church and suggesting an earlier construction phase of the Palace of the Columns.
A second research season in 2023 confirmed the results of the first season in the Church Group, and further expanded the research area to the Adobe, Arroyo, and South Groups.
The new findings from the second research season suggest the existence of an extensive network of subterranean tunnels and chambers under some of the major groups of structures that form the ancient site.
These results were obtained by a multidisciplinary team formed of 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 (ARX Project, A.C.).

Figure 1. Map of the archaeological site of Mitla with indication of the main groups of monuments and research conducted in the first and second season (Google Earth/ Marco M. Vigato)

Mitla, Gate of the Underworld

The archaeological site of Mitla is located in the central valley of Oaxaca, about 40 kilometers to the southeast of the state’s capital of Oaxaca. Mitla became the major political and religious center of the ancient Zapotec civilization after the fall of the earlier Zapotec capital of Monte Alban, in about 750 CE. Its greatest architectural development occurred in the Epiclassic and Postclassic period, between 750 and 1521 CE, with the construction of at least 5 main groups of structures named the Church Group, Columns Group, Arroyo Group, Adobe Group, and South Group.

While the earliest groups of structures, which include the Adobe Group and the South Group, followed the typical temple-plaza architectural distribution typical of Classic period constructions at Monte Alban, the more recent ones like the Church Group, Columns Group, and Arroyo Group, followed a different spatial distribution consisting of groups of rectangular structures around a central patio or plaza.

Although most of the structures visible today at the site date to the late Postclassic period (ca. 1250 – 1450 CE), evidence of human occupation at Mitla valley dates back at least 10,000 years, with a number of prehistoric caves having been discovered to the North of the site.

The Columns Group, perhaps the most famous ancient building at Mitla, consists of a large T-shaped structure named the “Palace”, located on a high platform facing a plaza with a central altar and elongated buildings to the East and West. The Palace is notable for its refined geometric mosaic fretwork ornamentation, obtained by the juxtaposition of thousands of small stone tiles fitted together without mortar. These stone friezes, known as “Grecas”, decorate both the outer and inner walls of the Palace, alternating a considerable variety of different patterns and designs, and are also found, although in a poorer state of preservation, in the other groups of structures at Mitla.

Two monumental tombs were uncovered by late 19th century and early 20th century excavations in the south plaza of the Columns Group. These consist of large cruciform chambers, cut directly from the natural bedrock, lined on the inside with carved stone panels and mosaic fretwork designs resembling those found in the upper rooms.

Another particularity of Mitla’s architecture is the use of enormous, finely cut and fitted megalithic stone blocks forming the bases of the walls and the lintels on top of the doorways. Some of these stones measure over 7 meters long and weigh more than 50 tons, coming from quarries located in a radius of several kilometers from the ancient site. Monolithic columns, over 5 meters high, were also used to support the roof of the largest chambers.

The Church Group, which occupies the northernmost part of the site, is so called because of the construction in the 16th century of a church dedicated to San Pablo Apostol on top and partially reusing the ancient structures.

The Arroyo Group resembles the Church Group in that it consists of three large patios surrounded by structures, and likely served as an elite residence.

The much more ruined Adobe (or Calvario) Group and South Group were built almost entirely of adobe and have lost their original stone facing and ornamentation. A small Catholic chapel, known as “Calvario”, was built in the late 16th century on top of the main pyramid of the Adobe Group.

According to a recent 2019 study, it is possible that a massive landslide which occurred towards the end of the Postclassic period was responsible for the destruction and abandonment of much of the site prior the Aztec and Spanish conquest.

The ancient buildings of Mitla were much admired by the early Spanish missionaries and historians of the Conquest and early Colonial period, who nevertheless demolished and looted the ancient structures of their stone to be used as construction material.

In 1674, Father Francisco de Burgoa left one of the most complete early descriptions of the ruins of Mitla. In his Geográfica Descripción, the Dominican priest describes the existence of a vast subterranean labyrinth under the site, considered by the ancient Zapotec to be an entrance to the underworld, or Lyobaa. According to Burgoa, four subterranean chambers existed under the main palace and residence of the high priests of Mitla. These chambers served as sanctuaries for the idols and contained the burial crypts of the high priests and kings of Teozapotlan. The last subterranean chamber connected with a vast cavern extending for an immense length under the ground:

“The last room had another door at the back, leading to a dark and terrifying space. It was sealed with a slab that covered the entire entrance… This terrifying cavern is why this town was named Liyobaa”

Living sacrifices were thrown into this dark cavern as offerings to the gods of the underworld. All entrances to this subterranean labyrinth were apparently sealed in the late 16th century with the construction of a Catholic church above the ruins.

“They all believed this dark cavern stretched more than thirty leagues beneath the earth, supported by pillars. Curious and zealous men and prelates attempted to dispel these ignorant beliefs. They descended through some steps with many people, carrying lit torches and lamps, until they encountered numerous buttresses resembling streets. They had brought ropes to guide them, for fear of losing their way in that confusing labyrinth. The stench and foul odor were as strong as the dampness of the ground, with a cold air that extinguished their lights. After a short distance, fearing being afflicted or encountering poisonous creatures (of which they saw some), they decided to retreat. They commanded to seal that infernal doorway completely, yet the upper rooms, with their courtyard and chambers, still remain.”

Since the mid-19th century, archaeologists have been searching in vain for the entrance to the underground labyrinth, the memory of which was never lost to the local people of Mitla. According to the ethnologist Howard Leigh, “For all Zapotecs the House of the Dead is the great subterranean labyrinth at Mitla, described by Burgoa, which archaeologists have been unable to find. The Mitla people claim that its entrance is hidden by the high altar of the Catholic Church”. (Leigh, 1960:3)

Project Lyobaa

In May 2022, a research team formed of specialists 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 (ARX Project, A.C.), obtained permission to conduct a comprehensive geophysical study of the ancient site of Mitla, with the objective of characterizing the subsoil structure of the site and to identify underground anomalies of potential archaeological interest, like tombs, subterranean chambers or tunnels. The research also aimed to provide valuable information for the management of seismic and geological risk to the ancient site.

To achieve its goals, the project employed state-of-the art, non-invasive geophysical research methods to create a complete 3D map of what lies underneath the site, focusing on its five main architectural groups. The research methods employed included Ground-Penetrating Radar (GPR), Electric Resistivity Tomography (ERT), and Ambient Noise Tomography (ANT). These methods measure the velocity at which different types of waves propagate across heterogeneous media and materials of different physical properties to detect changes in the structure and composition of the underground layers.

In the first 2022 season, research focused on the Church Group and the Columns Group, leading to the identification of significant voids and underground anomalies. Between the months of September and October of 2023, a second research season focused on the remaining groups of structures, the Adobe, Arroyo and South groups.

Results from the First Season (2022)

During the first research season, all three geophysical methods were employed for studying the Church Group and the Columns Group.

In the Church Group, a significant underground anomaly was detected under the church of San Pablo Apostol, extending in an East-West direction to a depth of over 6 meters (20 feet) below ground. This anomaly is indicative of the possible existence of a large chamber or a group of chambers, 15 meters long by 10 meters wide, with a floor-to-ceiling height of between 2 and 3 meters. This chamber appears to communicate with the surface by means of a shaft or tunnel located directly under the main altar of the Catholic church.

Figure 2. 3D image generated from Seismic scan of the Church group, at a frequency of 11.76 Hz. (ANT Image: Dr. Martin Cardenas Soto/ UNAM. Elaboration: Marco M. Vigato/ ARX Project).

Figure 3. 3D view of the same group of anomalies, showing location of Chamber A1 and A2 and of the connecting shafts. The entrance is located directly under the altar of the Catholic Church. (ANT Image: Dr. Martin Cardenas Soto/ UNAM. Elaboration: Marco M. Vigato/ ARX Project)

This chamber appears to connect with a second room located at a deeper level, which may be a natural cavern extending in a westerly direction to a considerable depth of over 15 meters (60 feet), beyond which the anomaly can no longer be followed.

A second and third set of tunnels and potential chambers were also detected to the North and to the East of the church, seemingly extending under the sacristy and connecting with more low-velocity and resistivity anomalies under the northern patios of the Church group. The roof of some of these chambers and tunnels appears to be located less than 1 meter (4 feet) below the present ground level and may thus be easily reached with traditional excavation methods.

In the Columns Group, both the Ground-Penetrating-Radar (GPR) and Electric Resistivity Tomography detected bodies of high resistivity under the present entrance stairway of the Palace. Other linear features were identified under the Palace and in the main plaza of this group. These may represent an earlier, buried stairway and the foundations of walls belonging to an earlier construction stage of the Palace. These findings are important for they suggest that the origins of this group may be older than previously thought, dating back perhaps to the Late Classic or Epiclassic period (ca. 750-900 CE).

In the Patio of Tombs, to the south of this Group, an unusual signal was picked up by the georadar at a depth of between 1 and 1.7 meters under the patio, near the entrance of Tomb 1, but extending in the opposite direction to that of this tomb.

Figure 4. GPR scan of the Palace in the Columns group, showing location of earlier stairway. (GPR Image: Dr. Denisse Argote Espino/ INAH. Elaboration: Marco M. Vigato/ ARX Project).

Latest Findings from the Second Season (2023)

In April 2023, a second research season was authorized by the INAH, with fieldwork taking place between the months of September and October of that same year.

The second season aimed to corroborate the findings from the first season in the Church Group with a different array of sensors, and to expand the research area to the remaining groups of structures in the Adobe (or Calvario), Arroyo, and South groups.

In the Church Group, the geophysical study was repeated with the use of a new array of 72 geophones, which allowed to obtain a better definition of the anomalies under the church, and to expand the research perimeter to the West and South. This new study confirmed the existence of a large East-West oriented cavity under the Catholic Church, as well as its possible connection to further low-velocity anomalies to the North, East and South. In the South, a significant anomaly was detected in the area between the Church Group and the Columns Group, which defines a possible East-West running tunnel or a system of chambers possibly connected to other large voids under the church.

Figure 5. 2D ERT profile taken from the modern ramp to the East of the Church Group, showing possible cavities and tunnels at depths between 3 and 12 meters below ground level. (ERT Image: Dr. Andrés Tejero Andrade/ UNAM. Elaboration: Marco M. Vigato/ ARX Project)

Figure 6. Seismic scan showing location of possible cavities and geophysical anomalies under the Church (Phase I, 2022) and in the area between the Church and the Columns Group (Phase II, 2023). Low-velocity anomalies (in blue) indicate potential cavities. (ANT Image: Dr. Martin Cardenas Soto/ UNAM. Elaboration: Marco M. Vigato/ ARX Project).

Figure 7. Location and orientation of potential cavities under the Church Group. The light blue/green color of the possible connecting tunnel between the north and south group of anomalies may suggest a partially filled or blocked passageway. (ANT Image: Dr. Martin Cardenas Soto/ UNAM. Elaboration: Marco M. Vigato/ ARX Project).

In the Arroyo Group, the ERT scan detected a filling of soft soil under the middle patio of this group (Patio H) extending down to a depth of approximately 2 meters below ground. Under this filling, close to the center of the patio, a well-delimited, low-resistivity area, may signal the existence at this point of a rock-cut chamber or shaft. Another anomaly compatible with a tunnel or shaft was detected under the southernmost patio (Patio I), at a depth of 2.7 meters (9 feet) below ground level.

Figure 8. ERT Scan of the middle patio (Patio H) of the Arroyo Group, showing location of possible subterranean chamber or tomb. (ERT Image: Dr. Andres Tejero Andrade/ UNAM. Elaboration: Marco M. Vigato/ ARX Project).

Figure 9. ERT scan of the southernmost patio (Patio I) of the Arroyo Group, showing a possible tunnel at a depth of 2.5 meters (A) connecting to a large chamber (B), possibly continuing further East. (ERT Image: Dr. Andrés Tejero Andrade/ UNAM. Elaboration: Marco M. Vigato/ ARX Project)

In the Adobe Group, both the ERT and Seismic Noise Tomography detected several potential large cavities associated with the pyramid to the East of this group and the adjoining plaza. A first cavity extends in a North-South direction under the base of the modern stairway leading up to the Calvario chapel on top of the pyramid. The seismic scan shows a possible entrance to this cavity near the Southeast corner, where an unusual monolithic stone lintel may be observed in the adobe filling of the platform. The ERT scan also shows possible cavities in this location. In the Plaza of the Adobe, facing the Calvario pyramid, some well defined anomalies were detected by ERT tomography to the east of the altar, which appear to form a set of subterranean chambers connected by tunnels, with two potential entrances to the North and South, approximately 35 meters (115 feet) apart. The two chambers, measuring approximately 3 by 4 meters (10 by 14 feet) across, appear to be of regular, rectangular shape, connected by angular tunnels at depths greater than 5 meters (18 feet) below ground.

Figure 10. ERT profile of the Calvario pyramid in the Adobe Group, showing location of possible cavity (A) in blue/ green color near the northeast corner. The color may suggest a partially filled in space. (ERT Image: Dr. Andrés Tejero Andrade/ UNAM. 3D model courtesy of Dr. Genevieve Lucet, Proyecto Mecate/ UNAM. Elaboration: Marco M. Vigato/ ARX Project).

Figure 11. ERT scan of the main plaza of the Adobe Group, west of the Calvario Pyramid, showing extensive tunnel system and clearly defined subterranean chambers at depths greater than 5 meters below ground. (ERT Image: Dr. Andrés Tejero Andrade/ UNAM. Elaboration: Marco M. Vigato/ ARX Project).

In the South Group, the seismic tomography scan showed several large low-density anomalies under the main mound (Mogote) of this group, apparently connecting below ground. This could be a partially filled natural cavity located at a depth of about 12 meters (40 feet) below ground. A possible connection to the surface may be observed to the South of the mound, in the form of a collapsed cave or tunnel entrance now filled with debris from the mound itself.

Figure 12. 2D Seismic profile of the South Group, showing location of possible cavities, appearing as low-velocity anomalies, in blue. (ANT Image: Dr. Martin Cardenas Soto/ UNAM. Elaboration: Marco M. Vigato/ ARX Project).

Figure 13. Profile cut of subsurface anomalies detected by the seismic scan at different depths under the Mogote pyramid, in the South Group. (ANT Image: Dr. Martin Cardenas Soto/ UNAM. Elaboration: Marco M. Vigato/ ARX Project).

Finally, an array of seismographs was deployed to the north of the archaeological site of Mitla over a distance of almost 3 kilometers (2 miles), which confirmed the presence of alternating layers of rock and soil likely originating from a massive landslide, thus confirming the hypothesis that much of the ancient site of Mitla was buried under a rock avalanche during historic times. Such an avalanche may explain the decline and partial abandonment of the site shortly before the Aztec and Spanish conquest, in the late 15th and early 16th century CE. It is possible that a considerable part of the ancient city lies buried to the North of the modern town of Mitla.

All these findings appear to substantially confirm the colonial accounts that speak of the existence of a vast network of tunnels forming an underground labyrinth under the ancient city of Mitla, considered by the Zapotecs to be an entrance to the Underworld.

More research needed to explore anomalies

In order to explore the geophysical anomalies identified in different parts of the archaeological site of Mitla, different solutions are being studied, including limited excavations and drilling with the insertion of micro-cameras or probes. With the use of specialized drilling equipment, it may be possible to perforate a small hole through the soil and rock layers to a depth of between 5 and 15 meters (18 to 40 feet), enough to insert a small camera, causing only minimal disturbance to the site.

These new probes may help to answer some of the still unresolved questions with respect to the geophysical anomalies identified: Are the anomalies connected? Are they natural or artificial? Are the tunnels and chambers accessible or are they filled in with debris or flooded?

The regular shape of some of the anomalies, and their predominantly East-West orientation, perpendicular to that of subterranean water streams, strongly suggests that they must be of artificial origin, or perhaps form part of an artificially modified natural cave system. The existence of a vast, unexplored cave system under the archaeological site of Mitla may explain why this site lacking natural resources or a particularly strategic and politically advantageous position, became one of the most important and sacred sites associated with the cult of the Dead in ancient Mesoamerica. Numerous prehistoric caves have been studied and documented in the mountains to the North and West of Mitla, containing extraordinary rock art and some of the earliest evidence of crop domestication in the entire American continent, dating back almost 10,000 years.

If a vast cave system does in fact exist under Mitla, as described by Spanish colonial sources, its importance for our understanding of the developmental sequence of ancient human cultures in the Valley of Oaxaca may be enormous.

In Memoriam

We wish to dedicate this research to the memory of Dr. Juan Esteban Hernández Quintero, who sadly passed away this past June 25, 2024, in the city of Morelia. It was a privilege to count with the participation and insights of Dr. Hernández Quintero during both phases of Project Lyobaa in 2022 and 2023. He is sure to leave an indelible mark in the memory of all of us who had the pleasure of working and collaborating with him. He is sorely missed by his family and colleagues.

Acknowledgments

We would like to express our most heartfelt gratitude to the entire project team:

Dr. Denisse Argote Espino (INAH), Arq. Leobardo Pacheco Arias (INAH), Dr. Andrés Tejero Andrade (UNAM), Dr. Martin Cárdenas Soto (UNAM), Dr. Gerardo Cifuentes Nava (UNAM), Dr. Aidé Esmeralda Lopez (UNAM), M.Sc. David Escobedo Zenil (UNAM), M. Eng. José Antonio Martínez González (UNAM), M.Sc. Juan Esteban Hernández Quintero (UNAM), Eng. Alejandro García Serrano (UNAM), graduate students Jesús Sánchez González, Sebastián Sotomayor Sandoval, José Felipe Viccón Hernández, Manuel Ortiz Osio, and Miguel Ángel Velasco Lindero.

Special thanks also to the personnel of the Mitla archaeological site, Corredor Arqueológico Vale de Tlacolula INAH-Covatlao, Centro INAH Oaxaca, Municipality of San Pablo Villa de Mitla, and Comité Calvario.

Thanks to Uriel Sánchez Sosa, Juan Carlos Zowy, and Don Ramiro for all their help and support throughout the project.

Also, a huge appreciation goes to the over 75 donors and sponsors who generously contributed to our fundraising campaign and who helped with their donations to cover for part of the costs of this second season of Project Lyobaa.

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