The first modern exploration of the great pyramid was carried out by Adolph Bandelier at the end of the 19th century. He unearthed some burials around the pyramid and drew plans of the structure.
Systematic archaeological investigations began in 1931, directed by architect Ignacio Marquina. After his success opening tunnels inside the pyramids of Teotihuacan, Marquina adopted this same method within the pyramid at Cholula in order to identify different construction phases, their dimensions, and dates.
From 1931 to 1950 under the direction of Marquina, and continuing in the 1960s by the "Proyecto Cholula", a total of 10 kilometers (~6 miles) of tunnels were opened inside the pyramid, criss-crossing several phases of superimposed buildings that made up this artificial mountain.
About 1 km (0.6 mi) of these tunnels is now open to visitors. The tunnels were cut into the earthen fill that covered the different construction phases, forming long corridors that narrow at the top into a kind of false-arch. Maybe for this reason, since they resembled the famous Maya false-arch or corbelled vault, they are often mistaken for ancient works and the visitors are puzzled by the question of why the ancient inhabitants of Cholula opened these tunnels into their sacred mountain!
