During the construction of the second phase of the underground, remains of the ancient Roman city Serdika were uncovered and are on display to the public, under a protective glazed roof. Laboriously uncovered and partially restored (not always without criticism), the complex covers an area of approximately 9000 square metres, which once included  eight streets, an early Christian basilica, six large buildings, mineral springs and a medieval church all located at underground level. Dating back to a period from 1st till 6th century AD, early examples of a water and sewage system, as well as ornately tiled floors have been uncovered.