In late 2003 a tunnel beneath the Temple of the Feathered Serpent was accidentally discovered by Sergio Gómez Chávez and Julie Gazzola, archaeologists of the National Institute of Anthropology and History (INAH).
After days of heavy rainstorm Gómez Chávez noticed that a nearly three-foot-wide sinkhole occurred near the foot of the temple pyramid.

First trying to examine the hole with a flashlight from above Gómez could see only darkness, so tied with a line of heavy rope around his waist he was lowered by several colleagues, and descending into the murk he realized it was a perfectly cylindrical shaft.
At the bottom he came to rest in apparently ancient construction – a man-made tunnel, blocked in both directions by immense stones.
Gómez was aware that archaeologists had previously discovered a narrow tunnel underneath the Pyramid of the Sun, and supposed he was now observing a kind of similar mirror tunnel, leading to a subterranean chamber beneath Temple of the Feathered Serpent.
He decided initially to elaborate clear hypothesis and to obtain approval.
Meanwhile, he erected a tent over the sinkhole to preserve it from the hundreds of thousands of tourists who visit Teotihuacán.
Researchers reported that the tunnel was believed to have been sealed in 200 CE.

Preliminary planning of the exploration and fundraising took more than six years.

Before the start of excavations, beginning in the early months of 2004, Dr. Victor Manuel Velasco Herrera, from UNAM Institute of Geophysics, determined with the help of ground-penetrating radar (GPR) and a team of some 20 archaeologists and workers the approximate length of the tunnel and the presence of internal chambers.
They scanned the earth under the Ciudadela, returning every afternoon to upload the results to Gómez’s computers.
By 2005, the digital map was complete.
The archaeologists explored the tunnel with a remote-controlled robot called Tlaloc II-TC, equipped with an infrared camera and a laser scanner that generates 3D visualization to perform three dimensional register of the spaces beneath the temple.
A small opening in the tunnel wall was made and the scanner captured the first images, 37 meters into the passage.

In 2009, the government granted Gómez permission to dig.
By the end of 2009 archaeologists of the INAH located the entrance to the tunnel that leads to galleries under the pyramid, where rests of rulers of the ancient city might have been deposited.
In August 2010 Gómez Chávez, now director of Tlalocan Project: Underground Road, announced that INAH's investigation of the tunnel – closed nearly 1,800 years ago by Teotihuacan dwellers – will proceed.
The INAH team, consisted of about 30 persons supported with national and international advisors at the highest scientific levels, intended to enter the tunnel in September–October 2010.
This excavation, the deepest made at the Pre-Hispanic site, was part of the commemorations of the 100th anniversary of archaeological excavations at Teotihuacan and its opening to the public.

It was mentioned that the underground passage runs under Feathered Serpent Temple, and the entrance is located a few meters away from the temple at the expected place, deliberately sealed with large boulders nearly 2,000 years ago.
The hole that had appeared during the 2003 storms was not the actual entrance; a vertical shaft of almost 5 meters by side is the access to the tunnel.
At 14 meters deep, the entrance leads to a nearly 100-meter long corridor that ends in a series of underground galleries in the rock.
After archaeologists broke ground at the entrance of the tunnel, a staircase and ladders that would allow easy access to the subterranean site were installed.
Works advanced slowly and with painstaking care; excavating was done manually, with spades.
Nearly 1,000 tons of soil and debris were removed from the tunnel.
There were large spiral seashells, cat bones, pottery, fragments of human skin.
The rich array of objects unearthed included: wooden masks covered with inlaid rock jade and quartz, elaborate necklaces, rings, greenstone croc
In late 2003 a tunnel beneath the Temple of the Feathered Serpent was accidentally discovered by Sergio Gómez Chávez and Julie Gazzola, archaeologists of the National Institute of Anthropology and History (INAH). After days of heavy rainstorm Gómez Chávez noticed that a nearly three-foot-wide sinkhole occurred near the foot of the temple pyramid.
First trying to examine the hole with a flashlight from above Gómez could see only darkness, so tied with a line of heavy rope around his waist he was lowered by several colleagues, and descending into the murk he realized it was a perfectly cylindrical shaft. At the bottom he came to rest in apparently ancient construction – a man-made tunnel, blocked in both directions by immense stones. Gómez was aware that archaeologists had previously discovered a narrow tunnel underneath the Pyramid of the Sun, and supposed he was now observing a kind of similar mirror tunnel, leading to a subterranean chamber beneath Temple of the Feathered Serpent. He decided initially to elaborate clear hypothesis and to obtain approval. Meanwhile, he erected a tent over the sinkhole to preserve it from the hundreds of thousands of tourists who visit Teotihuacán. Researchers reported that the tunnel was believed to have been sealed in 200 CE.
Preliminary planning of the exploration and fundraising took more than six years.
Before the start of excavations, beginning in the early months of 2004, Dr. Victor Manuel Velasco Herrera, from UNAM Institute of Geophysics, determined with the help of ground-penetrating radar (GPR) and a team of some 20 archaeologists and workers the approximate length of the tunnel and the presence of internal chambers. They scanned the earth under the Ciudadela, returning every afternoon to upload the results to Gómez’s computers. By 2005, the digital map was complete. The archaeologists explored the tunnel with a remote-controlled robot called Tlaloc II-TC, equipped with an infrared camera and a laser scanner that generates 3D visualization to perform three dimensional register of the spaces beneath the temple. A small opening in the tunnel wall was made and the scanner captured the first images, 37 meters into the passage.
In 2009, the government granted Gómez permission to dig. By the end of 2009 archaeologists of the INAH located the entrance to the tunnel that leads to galleries under the pyramid, where rests of rulers of the ancient city might have been deposited. In August 2010 Gómez Chávez, now director of Tlalocan Project: Underground Road, announced that INAH's investigation of the tunnel – closed nearly 1,800 years ago by Teotihuacan dwellers – will proceed. The INAH team, consisted of about 30 persons supported with national and international advisors at the highest scientific levels, intended to enter the tunnel in September–October 2010. This excavation, the deepest made at the Pre-Hispanic site, was part of the commemorations of the 100th anniversary of archaeological excavations at Teotihuacan and its opening to the public.
It was mentioned that the underground passage runs under Feathered Serpent Temple, and the entrance is located a few meters away from the temple at the expected place, deliberately sealed with large boulders nearly 2,000 years ago. The hole that had appeared during the 2003 storms was not the actual entrance; a vertical shaft of almost 5 meters by side is the access to the tunnel. At 14 meters deep, the entrance leads to a nearly 100-meter long corridor that ends in a series of underground galleries in the rock. After archaeologists broke ground at the entrance of the tunnel, a staircase and ladders that would allow easy access to the subterranean site were installed. Works advanced slowly and with painstaking care; excavating was done manually, with spades. Nearly 1,000 tons of soil and debris were removed from the tunnel. There were large spiral seashells, cat bones, pottery, fragments of human skin. The rich array of objects unearthed included: wooden masks covered with inlaid rock jade and quartz, elaborate necklaces, rings, greenstone croc
Teotihuacan