Archaeologists have spent the last 12 years exploring an 18-meter-deep tunnel beneath the Pyramid of the Sun in the ancient Mesoamerican city of Teotihuacan, 40 kilometers northeast of Mexico City. The tunnel was discovered in 2003 after heavy rain uncovered a hole near the pyramid temple. It extends 100 meters from the center of the square in front of the temple to the pyramid itself. The walls and ceiling are lined with pyrite, also known as fool's gold, which would have created a dazzling star-like effect in torch light.

The tunnel, which was sealed for 1,800 years, contained a treasure trove of ancient artifacts, including finely carved statues, jewelry, and ceramics, but most mysterious was the significant amount of liquid mercury found. Mercury is a rare liquid metal that is toxic to humans and very difficult to extract from deposits in the ground, meaning it is not found in large concentrations. The discovery has led to speculation that the metal was used for ritual purposes.