Other magnificent mosaics, recorded as covering the church interior, were lost either during the four centuries when it functioned as a mosque (1493–1912) or in the Great Thessaloniki Fire of 1917 that destroyed much of the city. It also destroyed the roof and upper walls of the church. Black-and-white photographs and good watercolour versions give an idea of the early Byzantine craftsmanship lost during the fire.
Following the Great Fire of 1917, it took decades to restore the church. Archeological excavations conducted in the 1930s and 1940s revealed interesting artifacts that may be seen in a museum situated inside the church's crypt. The excavations also uncovered the ruins of a Roman bath, where St. Demetrius was said to have been held prisoner and executed. A Roman well was also discovered. Scholars believe this is where soldiers dropped the body of St. Demetrius after his execution. After restoration, the church was reconsecrated in 1949.
Underneath the Church of St Demetrios is the place where St Demetrios, Thessaloniki’s patron saint, was martyred.
As the level of the ground gradually rose over the centuries, this area acquired the form of a crypt. According both to tradition and to archaeological findings, it was an old bathhouse, in which Demetrios was imprisoned and eventually martyred in ad 303. In the 5th century, when the first Church of St Demetrios was built, the site of his martyrdom was incorporated into the church and the fountain was converted into a source of holy water. In the years that followed, the fountain acquired basins, from which the faithful could collect myron, the sweet-smelling oil produced by the saint’s relics. The crypt filled up with earth during the period of Ottoman rule and was not rediscovered until after the fire of 1917. It has been restored by the Archaeological Service and was converted into an exhibition space in 1988.
It displays a collection of sculptures, capitals, closure slabs, and vessels from the Church of St Demetrios. More specifically, in room I there are sculptures from the original 5th-century church and piers with relief decoration and capitals with four acanthus leaves. In room II, in the saint’s chapel, there are inscriptions documenting the history of the church, together with figural sculptures of the Middle Byzantine period. Room III displays photographs, plans, and copies of the restoration work done on the church after the fire of 1917.
n the next room, room IV, there are sculptures from the decoration of the church which was built after the fire in the 7th century, and the ambo from the original 5th-century church is in room V. Rooms VI and VII, lastly, display sculptures from the decoration of the church in the Middle Byzantine period (10th century) and sculptures and pottery of the 13th–15th centuries. More specifically, these include the remains of the original ciborium, which was constructed to house first the saint’s icon and later his sarcophagus. The ciborium was hexagonal and made of wood and silver. There are also an arch and fragments of arches from a Byzantine ciborium over the altar, which latter is ornamented with crosses in medallions and crosses resting on orbs. An inscription indicates that the donor of the ciborium was Theodore, Bishop of Thessaloniki in the 13th century.