About 1,200 years ago, a reef in the middle of Lake Titicaca in what is now Bolivia became the repository of a people’s most valued possessions. In 2013, a sparkling cache of those objects was unearthed by underwater archaeologists. Six years later, researchers think they now know what the objects represent—evidence of a religion that helped the Tiwanaku state become a dominant force in the region.

Results of the excavation were revealed in a paper published today in the journal Proceedings of the National Academy of Sciences. Gold objects, metal ornaments, semiprecious stones, and incense burners recovered at the site suggest the reef—located near the Island of the Sun, home to multiple Tiwanaku sacred sites—was once used as a ritual site for the ancient state.

View Images Ritual offerings include gold, semi-precious stones, and incense burners adorned with pumas. Photograph by Teddy Seguin

Anthropologists are still piecing together details of the religion that helped make the Tiwanaku state, which existed between about 500 A.D. and 1,000 A.D. and extended to Chile and Peru at its height, so powerful. The Tiwanaku people didn’t leave behind significant traces of military might, and the state is thought to have amassed influence from religion and trade. And though archaeologists have discovered plenty of archaeological evidence of Tiwanaku religious beliefs, they’re still piecing together the meanings of the religion and how it may have contributed to the state’s expansion.

Artifacts found from the site, known as Khoa reef, include two gold medallions that represent Tiwanaku’s ray-faced deity and metal plaques that portray a mythical puma-llama hybrid. Divers also recovered the remains of real animals, including the bones of at least three young sacrificed llamas.

Another surprising find was five items made from Spondylus shells and one complete shell. The mollusks were important to early Andean cultures but are native to the Pacific Ocean, not Lake Titicaca. The fact that the shells were over 1,200 miles from their nearest habitat indicates both the Tiwanaku people’s trading relationships and the shells’ great value.

“Finding so many Spondylus was really remarkable,” says José M. Capriles, an anthropologist and assistant professor of anthropology at Penn State University and one of the paper’s authors.

Why did the Tiwanaku worshipers leave such valuable objects behind in the high Andes lake? Capriles sees the sacrifices as evidence of a religious tradition in the making—one that helped the Tiwanaku state grow and flourish. By using valuable, desirable materials in rites, Tiwanaku worshipers showed their commitment to their new religious traditions—customs that are “huge in terms of building societies,” Capriles says. “These deities that people are creating are becoming institutions that govern behavior.”

View Images A spondylus shell, center, was a precious object in the Tiwanaku state and an example of the valuable offerings left in the lake. Photograph by Teddy Seguin

That new religion set the groundwork for moral and behavioral norms. “If you behave well, you are immortal,” says Capriles. “But if you’re bad, you are going to get punished by the chief's deity.” It also meant that people could move from place to place, secure in the knowledge that their shared beliefs would keep them from being perceived as outsiders. This, the team posits, helped the Tiwanaku state expand.

At its height, the society had amassed significant political influence, economic power and cultural cachet. But after its collapse around 1,000 A.D., it was overshadowed by the cultures that came after it. “The Tiwanaku is the greatest Native American empire that many Americans have never heard of,” says Paul Goldstein, an archaeologist in the Department of Anthropology at UC San Diego who is also affiliated with the Scripps Center for Marine Archaeology. (Goldstein was not involved with the research.) “Every time we find something that reflects the complexity of the society, it adds to our deeper knowledge of the origins of complex societies worldwide.”