The offerings found at the base of the pyramid in the Teotihuacan ruin site just north of Mexico City include a green serpentine stone mask so delicately carved and detailed that archaeologists believe it may have been a portrait.
The find also includes 11 ceremonial clay pots dedicated to a rain god similar to Tlaloc, who was still worshipped in the area 1,500 years later, according to a statement by the National Institute of Anthropology and History, or INAH.
"We know that it was deposited as part of a consecration ritual for the construction of the Pyramid of the Sun," said INAH archaeologist Enrique Perez Cortes.
Experts followed an old tunnel dug through the pyramid by researchers in the 1930s that narrowly missed the center, and then dug small extensions and exploratory shafts off it. What they found points to the earliest days of the still largely mysterious Teotihuacan culture. The remains of three structures that predate the pyramid were found buried at the base. Archaeologists have known that the ceremonial significance of the site, perhaps as a "link" to the underworld, predates the pyramids.
They also found seven burials, some of them infant remains. Susan Gillespie, an associate professor of anthropology at the University of Florida who was not involved in the project, called the find "exciting and important, although I would not say it was unexpected" given that dedicatory offerings were commonly placed in MesoAmerican pyramids.
"It is exciting that what looks like the original foundation dedicatory cache for what was to become the largest (in height) pyramid in Mexico (and one of the largest in the world) has finally been found, after much concerted efforts looking for it," Gillespie wrote in an email.
She said the find gives a better picture of the continuity of religious practices during Teotihuacan's long history. Some of the same themes found in the offering are repeated in ancient murals painted on the city's walls centuries later.
George Cowgill, an anthropologist at Arizona State University, called the find "pretty important" and suggested the Tlaloc offerings may thicken the debate about whether the pyramid was linked to the sun, the underworld or Tlaloc, who was also considered a war god.
"The discovery of seven humans suggests that they were probably sacrificial victims, along with several species of fierce animals," Cowgill wrote.
The city was founded nearly 2,500 years ago and came to have a dominant influence in architecture, trade and cultural in large swaths of ancient Mexico. But the identity of its rulers remains a mystery, and the city was abandoned by the time the Aztecs arrived in the area in the 1300s and gave it the name Teotihuacan, which means "the place where men become gods."