The discovery of liquid mercury at the end of a tunnel beneath an ancient Mexican pyramid led archaeologists to believe they are close to discovering the tomb of a king. 

Deep inside the pyramid in the pre-Aztec city of Teotihuacan- one of the oldest cities in the Americas- archaeologist Sergio Gomez said he recently found "large quantities" of liquid mercury inside a chamber at the end of a tunnel, Reuters reported.

Though liquid mercury has been found in small quantities at other ancient sites in Mexico, it has never been found at Teotihuacan, a city of well-preserved stone pyramids and murals that thrived between 100 and 700 A.D. It's presence inside the Pyramid of the Feathered Serpent could mean scientists are closer than ever to finding the first ever royal resting place in the city. 

"It's something that completely surprised us," Gomez told Reuters. 

For the last six years, Gomez and a team of experts have dug their way through the tunnel that has not been opened in 1,800 years. They found thousands of artifacts, including jewelry, statues and seashells before coming across three chambers at the end.

Gomez isn't certain why the mercury, an element poisonous to humans, was found inside one of the chambers. One theory is it could have been used to represent a river leading to the underworld- much like the river Styx in Greek mythology, according to professor Annabeth Headrick of the University of Denver.

The metal would have been perfect for depicting a supernatural river due to it's mirror-like appearance, Headrick told The Guardian.

"Mirrors were considered a way to look into the supernatural world, they were a way to divine what might happen in the future," said Headrick, who's authored works on Mesoamerican art. "It could be a sort of river, albeit a pretty spectacular one." 

Teotihuacan, named after the Aztec word for "abode of the gods," is still an enigma despite decades of studies, research and excavations. The people who lived there remain nameless to modern researchers and no written record was left behind, including how the city was ruled.

According to Mexican archaeologist Linda Manzanilla, the possible tomb Gomez is working towards could belong to a lord, one of four that ruled the ancient city instead of a single king, Reuters reported. 

But no one will know until Gomez and his team finish the excavation sometime in October.

"It's still very uncertain, and that is what keeps everybody in suspense," American archaeologist George Cowgill, who has worked in Teotihuacan for 40 years, told the news agency.