Archaeologists have uncovered the remains of more than 150 skulls from an ancient shrine in central Mexico—evidence of one of the largest mass sacrifices of humans in pre-Hispanic Mesoamerica.

The skulls, many facing east, lay beneath a crude, slightly elevated mound of crushed stone on what was once an artificial island in a vast shallow lake, now completely dry.

"The site is barely a bump on the horizon in the middle of nowhere," said lead archaeologist Christopher Morehart, of Georgia State University. And that was baffling. Previous evidence of such sacrifices came from grand pyramids in large ceremonial centers.

The discovery suggests that the site—near the town of Xaltocan (named after the ancient lake)—played a significant role in the political turmoil during the period between the years 650 and 800. The great city of Teotihuacan, only nine miles (15 kilometers) away, had suddenly begun to collapse, and the power it once exerted over the region was slipping away. Many experts believe this turn of events was triggered by a massive drought.

What followed was a time of "political, cultural, and demographic change," according to Morehart, who has received funding from National Geographic's Committee for Research and Exploration. (National Geographic News is part of the National Geographic Society.)

As people left Teotihuacan and moved to the surrounding areas, new communities formed and new leaders competed for power. "There's a good chance that the sacrifices are related to these competitions," Morehart said.

The sacrificed individuals could even have been war captives—often the case in Mesoamerican cultures. The site itself was probably not a battlefield, though. It was a sacred space that was specially prepared for rituals.

The people who lived in this area appear to have performed elaborately choreographed rituals at the shrine before the fall of Teotihuacan, but they didn't include human sacrifice. Because of its water-bound location and the presence of freshwater springs nearby, the shrine was likely the site of ceremonies that petitioned gods associated with rain and fertility. Artifacts uncovered include clay images of Tlaloc, a rain god.

The rituals began to include sacrifices, though, as power struggles gripped the parched region. Morehart and his colleagues from the National University of Mexico believe that victims were first killed and dismembered. The body parts may then have been thrown into the lake, while the heads were carefully arranged and buried. Incense was burned during this ceremony, along with the resinous wood of pine trees. Flowers added their own perfume to the fragrant smoke, and foods such as ritually burned maize were presented as additional offerings.

Over the following centuries, new peoples arrived in the area and political power ebbed and shifted, yet the sacred nature of the site persisted. Morehart and his team found evidence for rituals here during both the Aztec and colonial periods, and they even came across a recent offering.