In 2003, flooding occurred in the Ciudadela (Citadel) of Teotihuacan and saltpeter began to damage the Temple of the Feathered Serpent. Work done to solve this problem led to one of the most important archaeological discoveries made in this site in recent years: an intact tunnel sealed for more than a thousand years. The project created to study the tunnel was named Tlalocan or Path to the Underworld. More than 60,000 objects have been recovered after years of exploration and removing huge amounts of soil and stones. This paper presents the first results of accelerator mass spectrometry (AMS) radiocarbon dating performed on some of those materials recovered from the tunnel. With these findings, in combination with the archaeological data, based on stratigraphy and ceramic typology, a chronology of several events is proposed concerning the construction phases and ceremonial use, as well as partial and definitive closures of the tunnel. Every closure was accompanied by a deliberate and structured deposition of offerings and ritual refuse along the tunnel. The range of ages that covers the Bayesian calibration of samples collected along the tunnel is around 115 yr, from AD 125 to 240. Material collected at the surface of the chamber located at the end of the tunnel and under the pyramid gave ages in the interval between AD 400 and 534. All samples analyzed fall within the interval of time that covers the period of occupation of Teotihuacan.