They had heard tales of thousands sacrificed at the Great Temple’s dedication, four rows of victims that stretched for miles, all waiting to have their hearts torn out.

The conquistadors and the Spaniards who followed them wrote of the victims of human sacrifices rolling down the steps of the temple, where they were dismembered, then eaten in a stew with chilies and tomatoes.

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But one thing terrified the European newcomers more than almost anything: A rack of human skulls that towered over one corner of the temple to Huitzilopochtli, the Aztec god of the sun, war and human sacrifice

Andres de Tapia, one of Hernán Cortés’s soldiers, wrote that were so many human skulls, he had to resort to multiplication to count them all.

“We found there were 136,000 heads.”

Those skulls, the conquistadors assumed, were what remained of men who had been defeated in battle.

They were both ornamentation and message: This is what happens to Aztec enemies.

Nearly 500 years later, scientists digging in Mexico City have unearthed the skulls.

They have also turned up more questions about the nature of Aztec human sacrifice that conflict with the conquistadors’ thinking.

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Their biggest finding: The skulls weren’t just the heads of male warriors who had been defeated by the Aztecs. Some were the smaller, thinner skulls of women and children.

“We were expecting just men, obviously young men, as warriors would be,” Rodrigo Bolanos, a biological anthropologist investigating the find, told the news agency Reuters, “and the thing about the women and children is that you’d think they wouldn’t be going to war.”

It’s clear the Aztecs had publicly displayed the skulls of women and children, but who were they?

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Defeated people from neighboring civilizations? Aztecs who had been sacrificed?

And why did the Aztecs display them in one of their holiest places?

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Researchers believe the tower of skulls was definitively a show of power by the Aztecs. But a more detailed explanation has eluded researchers and may have died with the Aztecs.

The skulls were found in the cylindrical edifice near Templo Mayor, one of the main temples in Tenochtitlan.

Bolanos and other researchers from the Mexican National Institute of Anthropology and History have been researching the skull rack since it was discovered in 2015.

The excavation unearthed nearly 700 skulls.

But the dig is ongoing, and researchers expect to find more as they get closer to the base of the tower of skulls.

The conquistadors weren’t exactly known for their attention to historical preservation. They slaughtered the Aztecs, who outnumbered the Spanish, but were literally outgunned. And the Aztecs who avoided Spanish bullets succumbed to Old World diseases, which further decimated the native population.

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On the ruins of the Aztec empire, Mexico City began to rise.

In fact, Cortés and the Spaniards who followed him used the pre-Hispanic structures as the foundation for new churches and cathedrals, according to the Associated Press.

It was both a symbolic decision and a practical one.

It showed how the Aztec gods had been displaced by the Christian church, but also saved the Spaniards the trouble of building new foundations, walls and floors.

Over the intervening centuries, forgotten Aztec ruins — and clues about their pre-Hispanic civilization — were buried beneath the largest city in North America.

But the ruins have refused to stay buried. Some were discovered in the rubble of buildings destroyed in a 1985 earthquake.

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And in 1978, workers laying electrical cables two blocks from the Zócalo, Mexico City’s main square, discovered the Aztecs’ Templo Mayor, or high temple.

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Centuries after the Aztec civilization fell, the surprise find is still yielding new artifacts — and raising new questions.

“Something is happening that we have no record of,” said Bolanos, the biological anthropologist. “This is really new, a first.”