Mithraeum in Jajce, in central Bosnia. Probably 4th century.
According to this blog (07/2012) "the Mithras Temple, the most ancient jewel in Jajce's sightseeing crown, was discovered by accident during construction. ... Now, it is in pieces, above ground, in a green tinted glass box by a condominium behind a Maxi supermarket. It's obviously in the process of being fixed up, completely moved from its original home to help stop the effects of moisture damage. A sign gives the estimated date of restoration completion as April of last year."
According to Unesco the monument was completed in December 2012; the Mithraeum was discovered in 1930 and required conservation after 80 years of neglect.
A detailed description is found here, from which the description is added below.
Official Bosnian description