What would make the discovery of a secret passageway under Teotihuacan’s Pyramid of the Moon even more intriguing? A theory that the tunnel was used to emulate the underworld, to start. CT scans performed by archaeologists from Mexico's National Institute of Anthropology and History in June indicate that a tunnel about 30 feet underground spans from the pre-Aztec pyramid to the middle of its outer central square, reports Live Science.