More than 10 years after it was discovered, a mysterious tunnel hidden under an ancient pyramid in central Mexico is giving up new secrets.

Archaeologists say they've found "large quantities" of mercury at the end of the 340-foot tunnel, which lies under the legendary Temple of the Feathered Serpent pyramid in the ancient city of Teotihuacan, some 30 miles northeast of Mexico City.

"It's something that completely surprised us," Sergio Gomez, the tunnel's excavation director, told Reuters, adding that his team does not yet know why the liquid metal was put there.