Archaeologists in China have discovered remnants of burned marijuana in a 2,500-year-old graveyard. “It’s the earliest strong evidence of people getting high,” said Mark Merlin, a botanist at the University of Hawaii.

Excavators found 10 wooden bowls with stones containing burnt residue from the cannabis plant. Scientists believe people used the stones to heat cannabis leaves, then inhaled the smoke as part of a burial ritual. The ancient pot was found in a complex of lofty tombs in the Pamir Mountains—a region near the borders of modern China, Pakistan, and Tajikistan.

The study’s scientists found evidence of THC, the compound that gives pot its high. Most wild cannabis plants have low levels of THC, so researchers believe the people who built the graves purposefully chose plants with higher amounts. “During funeral rites, the smokers may have hoped to communicate with the spirit world—or with the people they were burying,” said study co-author Yimin Yang of the University of the Chinese Academy of Sciences in Beijing.