The Great Basilica of Philippopolis (officially the Bishop's Basilica of Philippopolis) is the most significant early Christian monument in Bulgaria — a vast 4th-century church measuring 90 metres in length, whose remains lie in an open-air excavation site in the old town. At the time of its construction it was one of the largest Christian basilicas in the eastern empire.
The excavated ruins include substantial sections of the mosaic floor, column bases, and the outline of the three-aisled nave. The mosaics are among the finest surviving early Christian floor mosaics in the Balkans — intricate geometric patterns in blues, reds, and ochres that have survived 1,600 years underground. The site is open to visitors with explanatory signage.