The church of St Demetrios, the largest in Greece, is located in Salonika, the largest city in northern Greece. Built over a natural spring, the church became venerated by Orthodox pilgrims and Demetrios, depicted as a Roman soldier, became the patron saint of the city. Converted by the Ottoman Turks into a mosque c. 1493, its mosaics were plastered over. 

During repairs in 1907, the mosaics were rediscovered and photographed or drawn. In November 1912, the Greek army led by King Constantine occupied the city, liberating it from the Ottoman Empire, but an enormous fire inadvertently started by French troops in August 1917 destroyed much of Salonika including the church. Georgios Soteriou, the Ephor of Medieval and Christian Antiquities, was able to excavate below the floor which had subsided and rediscovered Roman remains in the crypt under the church.  In February 1922 Gilbert Bagnani attended a lecture at the Archaeological Society on the medieval monuments of Salonika given by Soteriou. His slides undoubtedly illustrated its recently destroyed mosaics as well as the buried crypt once frequented by early Christian pilgrims seeking the healing waters from the spring believed to be near Saint Demetrius’ burial site. Soteriou’s excavations under the ruined church discovered that the crypt had been fashioned out of a former Roman house bath near the ancient stadium, and filled in after the conversion of the church into a mosque in 1493.