Human sacrifice is the fact everyone knows about this empire and the hardest one to write about accurately. Within Mexica understanding the cosmos was not stable: four previous suns had ended, and the fifth was sustained only because the gods had thrown themselves into a fire at Teotihuacan, a debt that human offerings repaid. Captives taken in war, including in the arranged flower wars fought against neighbouring states for that purpose, were the ordinary victims, and the archaeological record at the Templo Mayor, including the skull rack excavated at its foot, confirms that the practice was real and organised.
The scale is another matter. The figure of eighty thousand at the temple dedication of 1487, repeated for centuries, is arithmetically impossible, and nearly all the narrative evidence comes through Spanish channels: Sahagun's Florentine Codex compiled from Nahua informants a generation after the conquest, Duran, and the accounts of conquerors who had every reason to describe what they had overthrown as monstrous. A separate argument ran through the 1970s over whether sacrifice served a nutritional function, and the ecological case was answered in detail and is not now generally accepted.
Tenochtitlan fell in 1521 and the temples came down, but Nahua religious life continued under Catholic forms, in the cult of saints, in the calendar of village fiestas and in the shrine at Tepeyac, and the excavated Templo Mayor now stands open in the middle of Mexico City.