(FOX NEWS) – Scientists may have discovered one of the oldest Christian churches in the world by using muon radiography to scan a mysterious subterranean building located in the ancient Russian city of Derbent, on the Caspian Sea’s coast.

Archeologists have long wondered exactly the purpose the building served and one hypothesis is that it was some type of Christian temple. Other possible theories for the building’s function: a reservoir or a Zoroastrian fire temple.

The 39-foot building, located in the northwest sector of the medieval fortress of Naryn-Kala, is almost completely hidden underground and built of local-shell limestone; it dates to about 300 A.D.

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