was digging in a well under the floor of a colonial-era home on the western side of the temple complex. Six feet under floor level, they discovered a wall of volcanic rock coated with stucco with a flagstone floor. The rectangular platform, estimated to be more than 34 meters (111.5 feet) long and 12 meters (40 feet) wide, has at its center a circular structure made from skulls cemented together using a lime, sand and volcanic gravel mortar. Many of the skulls have a hole 25 to 30 centimeters (10-12 inches) in diameter piercing the parietal bones. They are all facing inwards at the open space inside the circle. Adult male skulls predominate, but there are skulls from adult women, youths and children as well. So far archaeologistshave counted 35 skulls, but expect to see that number increase exponentially as they dig further down under the stucco and stone slabs.

Preliminary dating places this structure in Stage VI of the construction of the Templo Mayor (between 1486 and 1502), during the reign of Aztec warrior king Ahuízotl. He was succeeded on the throne of Tenochitlan by his nephew Moctezuma II who would meet his end fighting Conquistador Hernán Cortés. Cortés himself described the great tzompantli of Tenochtitlan, as did early ethnographers Franciscan friar Bernardino de Sahagún and Dominican friar Diego Durán. They wrote of tzompantli with low, elongated bases supporting the vertical posts with horizontal racks of skulls. There is also at least one account of skulls mortared together; this is the first time a tzompantli