Banya Bashi Mosque in Sofia, Bulgaria by Zbigniew

Banya Bashi Mosque is the only functioning mosque in Sofia, a remnant of the Ottoman rule of Bulgaria that lasted nearly five centuries. It is one of the oldest mosques in Europe, having been completed in 1576, during the years the Ottomans had control of the capital. The mosque derives its name from the phrase Banya Bashi, which means many baths. The most outstanding feature of the Mosque is that it was actually built over natural thermal spas. One can even see the steam rising from vents in the ground near the Mosque walls (wiki).