When the Spaniards arrived in Tenochtitlan in 1519, the Aztec capital’s main shrine stood 150 feet high. Little still stands of that building today because the Spaniards demolished it and used its blocks to build their own cathedral, known as the Metropolitan Cathedral of the Assumption of Mary, within sight of the remains of the once soaring temple. Possibly unknown to the Spaniards, however, at least six earlier versions of the Templo Mayor still lay underneath the structure they destroyed, the result of each successive ruler building his own temple on top of the previous one.

Since the early 1980s, archaeologists have been delving into those earlier layers, gaining a look at how the Aztecs worshipped decades before the conquest. Because these remains had been buried since the 1400s, they are giving researchers an unprecedented look at classical Aztec society. One of the first artifacts they excavated was a monumental stone disk dating from an early phase of the temple’s construction, around 1400, depicting the moon goddess Coyolxauhqui, a figure from the Aztec creation myth. In the legend, the goddess was decapitated and dismembered at the hands of her brother Huitzilopochtli as punishment for disrespecting their pregnant mother. Archaeologists have concluded from the chopped-off human limbs and heads excavated near the temple’s base that the grisly scene was reenacted regularly at Huitzilopochtli’s altar on the summit. Rows of skulls made of stone and stucco, still visible today, had their counterparts in actual skulls excavated nearby.

The carnal nature of Aztec worship has long intrigued researchers, in part because its focus on blood-drenched sacrifice in the public square had few parallels in other Mesoamerican societies. Scholars suggest that the elites may have felt insecure in their power, and responded with these grandiose, intimidating rituals. “You get a sense of who ran society and how they made themselves loom large over it, monumentalizing themselves, and how they expressed power with these acts,” says Harvard University historian David Carrasco. Sacrifice was also closely linked to warfare—the victims were mostly battlefield captives—and thus to economic domination over neighboring states, explains archaeologist Eduardo Matos Moctezuma.

The greatest Aztec conqueror of them all, Ahuítzotl, was cremated upon his death in 1502 and his ashes placed in an urn at the base of the temple, according to sixteenth-century accounts. Archaeologists thought they might be close to finding his remains in 2006 when they excavated a stone inscribed with the year 10 Rabbit in the Aztec system (which corresponds to A.D. 1502) along with artifacts suggesting an elite burial. They now think that the urn with Ahuítzotl’s ashes had actually been dug up in 1900 by Mexican archaeologist Leopoldo Batres, who did not know he’d struck the Templo Mayor. At that time, the neighborhood around the buried ruins had few houses and a reputation for bad omens and ill spirits, likely a remnant of the site’s bloody history, says archaeologist Raúl Barrera.