IN a discovery set to delight alien invasion enthusiasts, 1800-year-old, once-metallic orbs have been found under an ancient pyramid in Mexico City.

The spheres, covered in pyrite, or fool's gold, are interesting on their own but the fact that they were found deliberately buried under a pyramid in the ancient city of Teotihuacan, once the most populated city in the world, has led some to call it an "unprecedented discovery."

Anthropologists deduce that the Teotihuacans knew they were not going to survive and mysteriously abandoned the city in 700AD. Some say that may have been due to a famine or due to some kind of invasion.

Mexico's National Institute of Anthropology and History has posted photos of the discovery on its website. See the images below.

Before the Teotihuacans disappeared they hid the orbs at the Temple of the Feathered Serpent in tunnels so deep that it took scientists years of planning before they could even dig.

Gizmodo reports that wireless robots are currently scouring the area for any new discoveries with researchers still having three chambers left to dig through.

Scientists are still in the dark about what the orbs may have been used for but possibly they were used for religious purposes to present to the gods.

The orbs were yellow, which comes from jarosite, which forms as pyrite oxidizes. When the orbs were used, they would have been glittering balls of gold.

Opening the Gates of Hell

The discovery comes a month after group of Italian archaeologists have announced they have found the legendary "Pluto's Gate", a portal filled with foul-smelling noxious fumes which inflicted a quick death on any person or beast that was driven into its embrace.

The temple complex in Hierapolis, now the volcanic-spring restort town of Pamukkale, featured in many ancient legends and historical texts.

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Originally published as Mystery of pyramid's golden balls