Imagine going swimming in the (now drained) lake, not knowing what lay below. Via Live Science:
Archaeologists have unearthed a trove of skulls in Mexico that may have once belonged to human sacrifice victims. The skulls, which date between A.D. 600 and 850, are “potentially evidence of the largest mass human sacrifice in ancient Meso-America.”
[The site is] in a now drained lake called Lake Xaltocan. To date, more than 150 skulls have been discovered there, as well as a shrine with incense burners, water-deity figurines and pottery suggesting a ritual purpose.
The findings shake up existing notions of the culture of the day, because the site is not associated with Teotihuacan or other regional powers. The shrines and the fact that sacrifice victims were mostly male suggest they were carefully chosen, not simply the result of indiscriminate slaughter of a whole village.