Mithraeum I was discovered in 1898 and 1899 and has been preserved in situ (on the spot where it was discovered) in the settlement of Spodnja Hajdina. The temple was built in the 2nd century AD by the officials working for the Illyrian customs station which had its headquarters in Poetovio. The temple was dedicated to god Mithras who descended to earth in order to create a new world.

The building has three naves and the deepened central part where are to be found monuments dedicated to god Mithras by his worshippers, the rock from which Mithras was born, as well as monuments dedicated to Mithras’ two assistants, genies, Cautes and Cautopates, as can be deduced from inscriptions on the monuments. Among the most significant monuments in Mithraeum I are the statue of Mithras’ birth from the rock and Mithras dragging a bull – Taurophoros.