showed interest in two nearby mounds called the Cerro de Acozac and the Cerro de la Cruz, which at that time were totally covered in vegetation. The pyramid was excavated in two phases. The first began in 1931 and ended in the 1950s. Excavations began again in 1966 and officially ended in 1970, with the publication of the reports of the various academics who worked on Proyecto Cholula. Exploration of the pyramid itself began in 1931 under architect Ignacio Marquina who dug tunnels to explore the substructures. This was a time of political instability in Mexico, mostly due to the Cristero War. However, the decision was made to excavate the structure because of the success of the excavations of Teotihuacan by Manuel Gamio in the 1920s. The project was given to Marquina because of his experience working with Gamio in Teotihuacan. While credit is given to Marquina, he spent little time at the site. Most of the work was really done by Marino Gómez, the site guardian, who directed the digging of the tunnels. These tunnels allowed for the mapping and modelling of the pyramid's successive layers. The pyramid had no obvious entrance, due to its deteriorated condition, but the archaeologists decided to begin tunnelling on the north side, where colonial construction had damaged it. On this side, the remains of walls and other structures could be seen. Tunnelling techniques were based on various team members' experience at Teotihuacan and Tenayuca. These techniques were based on mining, even using small tracks and miniature coal cars to carry out debris. Examples of these are at the site museum. The base of the pyramid was constructed of sun-dried adobe bricks, which contained ceramic, obsidian and gravel for better compacting. This solid foundation allowed the excavators to only need to create "false arches" like those found in Mayan constructions, rather than adding beams and other supports. The first two tunnels were built to criss-cross the centre, one north–south and the other east/west. When the tunnels reached substructure, they then followed the contour of the structure, and eventually the various tunnels created an underground labyrinth. By 1939, there were four kilometers of tunnels, with two more added by 1951. The tunnels demonstrate the real value of the pyramid, which is not visible on the surface. During the first round of excavations, sixteen holes were dug in the area by Eduardo Noguera to extract ceramic