A Mexican archaeologist hunting for a royal tomb in a tunnel beneath a towering pre-Aztec pyramid has made a discovery that may have brought him a step closer: liquid mercury.

In the bowels of Teotihuacan, a mysterious ancient city that was once the largest in the Americas, earlier this month Sergio Gomez found 'large quantities' of the silvery metal in a chamber at the end of a sacred tunnel sealed for nearly 1,800 years.

And because of the potential supernatural significance of liquid mercury in ritual ends, deeper into the complex Gomez hopes to find the elusive last resting place of a king.

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Hope: Mexican archaeologist Sergio Gomez has been excavating a pre-Aztec pyramid in Teotihuacan, Mexico, for six years and came across 'large quantities' of liquid mercury earlier this month

Underground: Gomez announced on Friday that he found the liquid mercury in a chamber at the end of a tunnel (pictured) that had been sealed off for more than 1,800 years

'It's something that completely surprised us,' Gomez said at the entrance to the tunnel below Teotihuacan's Pyramid of the Plumed Serpent, about 30 miles northeast of Mexico City.

If Gomez is right, it could help settle a debate over how power was wielded in Teotihuacan, a city boasting massive stone pyramids that was home to as many as 200,000 people and the heart of ancient empire that flourished between 100 and 700 A.D.

Some archaeologists believe the toxic element could herald what would be the first ruler's tomb ever found in Teotihuacan, a contemporary of several ancient Maya cities, but so shrouded in mystery that its inhabitants still have no name.

Unsure why the mercury was put there, Gomez says the metal may have been used to symbolize an underworld river or lake.

Though mercury has previously been uncovered in small amounts at a few Maya sites much further south, it had never been found in Teotihuacan.

Mercury was rare in ancient Mexico, as it was difficult to mine and prized for its reflective properties. Archaeologists believe it may have had a supernatural significance for ritual ends.

Teotihuacan, or 'abode of the gods' in the Aztec language of Nahuatl, was distinct from the Mayan civilization.

Royalty: Gomez said that he is unsure as to why the mercury was in the tunnel (pictured), but he hopes that it will lead him to the tomb of the city's king

Digging: National Institute of Anthropology and History (INAH) archaeologists work at a tunnel that may lead to a royal tombs discovered at the ancient city of Teotihuacan in 2011. Gomez said their findings could lead to settling the debate as to how power was distributed in the city

Its inhabitants left behind no written record, abandoning the city long before the Aztecs came to power in the 14th Century.

Spaniards dug at Teotihuacan in the 1670s, but rigorous scientific excavation of the site did not begin until the 1950s.

Gomez's six-year slog in the tunnel has already yielded tens of thousands of artifacts including stone sculptures, fine jewelry and giant seashells leading to the three chambers.

The painstaking excavation has slowed due to extreme humidity, mud, and now, the need for protective gear to avoid mercury poisoning.

George Cowgill, a U.S. archaeologist who has spent more than four decades excavating Teotihuacan, says the mercury find increases the odds of finding a tomb.

'But it's still very uncertain, and that is what keeps everybody in suspense,' he added.

Mexican archaeologist Linda Manzanilla believes that at its peak, the city was ruled not by a single king, but a council of four lords, and that Gomez may find the remains of one of them.

To bolster her argument, she cites the absence of a single palace or the presence of kings in any of the city's many murals.