Divers scouring the bed of a lake high in the Andes Mountains have found prehistoric deposits of gold, gems and a sacrificed baby llama — evidence of rituals that may have helped leaders to strengthen their hold over an expanding empire.

The Tiwanaku civilization took root some 1,500 years ago next to Lake Titicaca, which sits on the border of modern Bolivia and Peru, and eventually came to dominate a large swathe of the region. Christophe Delaere at the Université Libre de Bruxelles, José Capriles at Pennsylvania State University in University Park and Charles Stanish at the University of South Florida in Tampa studied items placed in the lake between roughly AD 800 and 950, when Tiwanaku power and influence were spreading.

The objects include a turquoise pendant, seashells imported from the distant Pacific Ocean and a gold medallion depicting a Tiwanaku deity. Such sumptuous offerings would have advertised Tiwanaku leaders’ authority and wealth. The ceremonies also promoted religion, which can bind together far-flung peoples and encourage cooperation.