During a years-long exploration of the ground beneath the Temple of the Feathered Serpent in the ancient city of Teotihuacan in Mexico, archaeologists have discovered liquid mercury down a tunnel found there a decade ago.

The reason for the existence of the toxic substance is unknown, though it is commonly thought that it may have ritualistic significance, such as a symbolic "river to the underworld."

The hope is that the discovery will lead to the uncovering of the tomb of one of the city's rulers. Since none have been found so far, the nature of Teotihuacan's government is still largely a mystery.

Teotihuacan thrived between the first and seventh centuries A.D. It covered roughly fourteen square miles, with a population over 100,000, quite large for that era.

Check out the 45-minute documentary on ancient Teotihuacan, known as the "city of the gods," here: