Mexico’s Huei Tzompantli, a terrifying tower constructed from human skulls, is even bigger than previously thought. Archaeologists have discovered a new section of the historic site, which was first uncovered in 2015. The new find adds 119 skulls to the previously known 484, for a hair-raising total of 603 skulls, stacked atop one another and mortared together.

The tower is believed to be one of seven similar structure that once stood in the Aztec capital city of Tenochtitlán (now Mexico City). The skull towers are mentioned in contemporary accounts of the city’s capture by Hernan Cortes in 1521, and were razed by the Spanish conquistadores shortly thereafter.

Archaeologists have long known that Mesoamericans displayed the severed heads of captured warriors on skull racks, or tzompantli in the Aztec language of Nahuatl. But the new discovery included the skulls of three children, identified by their immature teeth, as well as women’s heads.

“We were expecting just men, obviously young men, as warriors would be, and the thing about the women and children is that you’d think they wouldn’t be going to war,” Rodrigo Bolanos, a biological anthropologist working on the site, told Reuters. “Something is happening that we have no record of, and this is really new, a first in the Huey Tzompantli.”

The Huey Tzompantli, a “tower of skulls” erected by the Aztec in Tenochtitlán. Photo courtesy of Mexico’s National Institute of Anthropology and History.

The Urban Archeology Program at Mexico’s National Institute of Anthropology and History is carrying out the dig. Unearthed near Mexico City’s Metropolitan Cathedral, itself built over the 14th- and 15th-century ruins of the Aztec religious center Templo Mayor, the cylindrical structure measures over 16 feet in diameter.

“The Templo Mayor continues to surprise us, and the Huey Tzompantli is without doubt one of the most impressive archaeological finds of recent years in our country,” said Alejandra Frausto, Mexico’s culture minister, in a statement.

Archaeologists believe the tower was constructed between 1486 and 1502, and that the skulls belonged to victims of ritual sacrifices to the gods.

“They were all made sacred,” said archaeologist Raul Barrera in a statement. “Turned into gifts for the gods or even personifications of deities themselves.”

See more photos of the discovery below.

Archaeologists excavating Huey Tzompantli, a “tower of skulls” erected by the Aztec in Tenochtitlán. Photo courtesy of Mexico’s National Institute of Anthropology and History (INAH).

The Huey Tzompantli, a “tower of skulls” erected by the Aztec in Tenochtitlán. Photo courtesy of Mexico’s National Institute of Anthropology and History.

The Huey Tzompantli, a “tower of skulls” erected by the Aztec in Tenochtitlán. Photo courtesy of Mexico’s National Institute of Anthropology and History.