A 600-foot tunnel below a pyramid in the ancient Mexican city of Teotihuacan has given up 50,000 artifacts—but its latest reveal could indicate something far more significant lies in wait.

After six years of toiling below the Pyramid of the Plumed Serpent, archaeologist Sergio Gomez says he's uncovered "large quantities" of liquid mercury at the end of the tunnel, which leads to three chambers that have been untouched for 1,800 years.

Though the reflective metal has been found at Maya sites to the south of Teotihuacan, outside Mexico City, "it's something that completely surprised us," Gomez tells Reuters.

Rare in ancient Mexico, liquid mercury may have been used to symbolize a lake or river that would carry a spirit to the underworld and experts suggest an unexplored chamber may house the first royal tomb ever discovered in Teotihuacan.

"Mirrors were considered a way to look into the supernatural world, they were a way to divine what might happen in the future," an archaeologist explains, per the Guardian.

"It could be a sort of river, albeit a pretty spectacular one." Teotihuacan, separate from the Maya civilization, rose to prominence 1,900 years ago and became the largest city in the Americas before it was abandoned around 700AD.

As no written record was left behind, archaeologists know little about the city and the 200,000 people who probably once called it home. While Gomez hopes a king's tomb will give insight into Teotihuacan's system of power, excavations have revealed no royal residence and the city's various murals don't portray a single ruler, an archaeologist notes, suggesting Gomez may instead find a tomb of one of four lords who may have shared rule.

Gomez says a study of the tombs should wrap up in October. (Click for more Teotihuacan discoveries.)

This article originally appeared on Newser: Teotihuacan Find Boosts Hope of Royal Tomb

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