Researchers from Mexico's Nation University found the tunnel, which lies under the ancient city of Teotihuacan, with radar detection. The 340-foot tunnel leads to several funeral chambers, which the researchers suspect may be the final resting places of ancient rulers. Archaeologists first began exploring the tunnel in 2010 using a mini-robot.
Sacrificial offerings in front of the burial chambers were clues that the occupants were likely royalty. The finds from the tunnel include stone sculptures, jewelry, arrowheads, and obsidian blades, along with thousands of other ritual objects.
The city's inhabitants left no written records, so the tombs could provide historians with clues about how their ruling system worked. Teotihuacan, about 30 miles north of Mexico City, had a population of about 200,000 between 650 and 100 B.C.E., and it "dominated central Mexico in pre-Columbian times," the BBC notes. The team will continue their research in the tunnel's chambers next year.