Archaeologists had previously found a few hundred skulls in the the Aztec capital Tenochtitlan the site of present day Mexico City, however ongoing research has found this was only the tip of the iceberg and the human sacrifices numbered in the thousands.
Archaeologists were aware the Aztec performed human sacrifices, but the truth of how many were killed in the most horrific fashion became undeniable in 2015. Mexico's National Institute of Anthropology and History (INAH) found what was known as a "trophy rack" near the the site of the Templo Mayor, which is present Mexico City.