A Mexican archaeologist hunting for a royal tomb in a tunnel deep beneath a 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, Sergio Gomez this month found "large quantities" of the silvery metal in a chamber at the end of a sacred tunnel sealed for nearly 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 (50 km) northeast of Mexico City.