Archaeologists excavating beneath Mexico’s Temple of the Feathered Serpent have discovered hundreds of mysterious yellow orbs.

Tunnels near the third largest pyramid in the pre-Hispanic city of Teotihuacan have been the focus of archaeological study ever since they were discovered in 2003.

The yellow spheres were uncovered when a remote-controlled robot carrying camera equipment was deployed to explore a series of winding and largely inaccessible chambers within the ancient pyramid ruins that are characterised by statues of strange serpent-like creatures.

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"They look like yellow spheres, but we do not know their meaning," Jorge Zavala, an archaeologist at Mexico's National Anthropology and History Institute, told ABC news of the find. "It's an unprecedented discovery."

The orbs measure between 1.5 and 5 inches and are believed to be covered in a yellow material called jarosite and to contain a core of clay.

The World Heritage Site, a city of pyramids located just 30 miles from Mexico City, is thought to have been established around 100 B.C and was inhabited by around 100,000 people at its peak before being mysteriously abandoned around 700 A.D.

The remote-controlled robot Tlaloc II-TC sent to explore the tunnels carries an infrared camera and a laser scanner that generates 3-D visualisation of the spaces beneath the temple, allowing it to access parts of the ruin which have not yet been excavated.

"A few months ago we found two side chambers at 72 and 74 metres from the entrance. We called them North Chamber and South Chamber,” archaeologist Sergio Gómez Chávez, director of the Tlalocan Project, told Discovery News.

"The robot was able to enter in the part of the tunnel which has not yet been excavated yet and found three chambers… We believe that high-ranking people, priests or even rulers, went down to the tunnel to perform rituals.”

George Cowgill, professor emeritus at Arizona State University, told Discovery News the find was “unique”.

He said: “Pyrite was certainly used by the Teotihuacanos and other ancient Mesoamerican societies. Originally the spheres would have shown brilliantly. They are indeed unique, but I have no idea what they mean.”

The walls of the tunnels are covered in a mineral powder made up of magnetite, pyrite and hematite. Gomez believes the tunnel was sealed twice by the Teotihuacan people and the access was blocked nearly two millennia ago in order to project something very important in the central chamber.

He believes the tunnels might contain the remains of those who ruled Teotihuacan and that the site is possibly one of the most significant archaeological finds in the region.