The St. Sofia Basilica is located in the downtown of the Bulgarian capital Sofia, and is said to be the oldest functioning church in Europe. It is a cross basilica with three altars featuring Early Christian ornamental or flora and fauna-themed floor mosaics. It was first built in the 4th century AD on the site of several earlier places of worship, which was also the site of the necropolis of the Ancient Thracian and Roman city of Serdica, even though it was rebuilt in the 6th century into its current form. This is believed to be the fifth structure built on the site. Its last reconstruction was during the reign of Byzantine Emperor Justinian I (r. 527-565 AD), which makes it a contemporary of the Hagia Sophia Chruch in Constantinople, today’s Istanbul.
The St. Sofia Basilica is also famous for hosting the Early Christian Council of Serdica, probably held in 343 AD, and attended by 316 bishops. During the Second Bulgarian Empire (1185-1396 AD), it gained the status of a metropolitan church, and in the 14th century it gave its name to the city which had been known by its Slavic-Bulgarian name of Sredets. In the 16th century, after Bulgaria’s conquest by the Ottoman Turks, the St. Sofia Basilica was turned into a mosque with the destruction of the 12th century murals and the addition of minarets. However, two earthquakes destroyed one of the minarets in the 19th century, and the mosque was abandoned. It has been restored since 1900. In 2013, the ancient necropolis and tombs underneath the St. Sofia Basilica, some of which have murals, were turned into an underground museum open for visitors.