Few of the modern visitors to Teotihuacan are aware of the vast and mysterious underworld of caves and man-made tunnels that extends under much of the ancient site and for miles around. The existence of these tunnels has been known for centuries, but not even the most recent research has been able to solve the mystery of their origin and purpose. Very much like at Giza, in Egypt, these tunnels are rumored to connect all the main pyramids by means of underground passageways, and perhaps even lead to the records of a lost civilization.
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The French explorer and anthropologist Desiré Charnay was among the first Europeans to penetrate into this labyrinth of tunnels in modern times, and leave a detailed account of it. In his 1880 book “ Les anciennes villes du Nouveau Monde ”, Charnay recounts having been led by his guide to some cavernous quarries, two-and-a-half miles (1.6 kilometers) west of the Pyramid of the Moon. There he was showed the entrances to several galleries, branching off in different directions at regular angles. These led to different chambers, which he described as “large halls”, one in particular “shaped like a rotunda” and filled with human remains. Charnay had no doubt that these tunnels had been dug in ancient times, and he speculated these could have served as quarries to build the many structures above ground in Teotihuacan, and were only later turned into catacombs. Charnay followed another tunnel for a distance of almost one kilometer (0.62 miles), without approaching its end. The tunnel never deviated from its course and appeared to point in the direction of the Pyramid of the Sun, almost two kilometers (1.2 miles) further to the south-east. Interestingly, these tunnels seemed to be partially dug into the volcanic conglomerate that covers the entire valley to a depth of several meters, and part into the much harder bedrock.

Panoramic view of the pyramid of the Sun, Teotihuacan, Mexico. (Mariordo (Mario Roberto Durán Ortiz)/ CC BY-SA 4.0 )
A local legend collected by Peter Tompkins in his “ Mysteries of the Mexican Pyramids ”, has one of these caves running on a straight line to Amecameca, some 65 km (40 miles) to the south-east. The existence of these subterranean caves was certainly well known to bandits and revolutionaries during the 1800s and early 1900s, as many of them allegedly took refuge in the caves at the time.

View of the “Cueva del Pirul”, one of the largest systems of interconnected caves to the East of the Pyramid of the Sun. One can notice the many rough pillars left to support the roof and a number of side passages branching out in different directions. (Photo by author Marco M. Vigato).
Quarries and just quarries?
Geologists believe that this set of cavities originated millions of years ago, when intense volcanic activity in the area left a network of “ lava tubes ” and bubble-shaped caves, some as much as 20 meters (65.6 feet) high and up to 100 meters (328 feet) in length. The ancient inhabitants of Teotihuacan certainly enlarged and expanded these natural cavities as a source of construction material. Millions of tons of volcanic rocks were extracted from these tunnels, although it is unclear why the ancient Teotihuacanos would not have chosen the much more accessible deposits above ground. Rather, they chose to dig a maze of tunnels in the near complete darkness, under constant threat of cave-ins and floods.
Already over the course of the 1950s, French-American archaeologist René Millon speculated that the largest pyramids could have been built on top of vast underground cavities. Digging under the Pyramid of the Sun, he found evidence of a huge blocked pit, which he believed could lead to a tomb “of immense proportions.” It was not until 1971, however, that excavations revealed the entrance to an ancient tunnel underneath the pyramid. The passage ran in an easterly direction for about 100 meters (328 feet), until a system of chambers arranged in the shape of a four-leaf clover, very near the center of the pyramid. The tunnel appeared to have been deliberately blocked and sealed in antiquity, but the chambers were found to be completely empty except for a few smaller obsidian artifacts and pottery fragments. No trace of the “huge pit” reported by Millon was found.