Human skulls unearthed beneath the heart of Mexico City has raised new questions about the culture of sacrifice in the Aztec Empire after crania of women and children surfaced among the hundreds embedded in the forbidding structure.

Giving a glimpse, perhaps, into a practice of human sacrifice by the Aztecs, archaeologists working in Mexico City have discovered more than 650 skulls.

The skulls were encrusted in lime near a main temple in the Aztec capital Tenochtitlan, as Mexico City formerly was known.

The crania include those believed to have been of children and women, Reuters reported.

Experts say the tower of human skulls evidently is linked to Huey Tzeompantli, which horrified Spanish conquerors when, let by Hernan Cortes, they took over the city.

Prior to the Spanish conquest, severed heads of defeated warriors were displayed on skull racks, Reuters said.

“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,” said Rodrigo Bolanos, a biological anthropologist investigating the find, to Reuters.

“Something is happening that we have no record of, and this is really new, a first in the Huey Tzompantli,” he added.

Raul Barrera, an archaeologist who worked at the site, believes the skulls were placed in the tower after being displayed on the tzompantli.

Barrera expressed certainty that the tower is one of several skull groups discussed by a Spanish soldier who accompanied Cortes in the 1521 conquest of Mexico, Reuters reported.

Barrera expects the number of skulls to rise as the excavations continue.

Human sacrifices were a ritual of the Aztecs and other Mesoamerican peoples who used the practice to make offerings to the sun.