The Crypt of St. Demetrius is a catacomb located under the Church of St Demetrios where St Demetrius, Thessaloniki city’s patron saint, was apparently martyred. Archaeological findings have revealed that the crypt was actually an old bathhouse where Demetrius was kept imprisoned and eventually died in 303 AD. The church was then built in the 5th century at the site of his martyrdom, which was incorporated and the fountain was converted into a source of holy water.

The crypt started filling up with dirt during the Ottoman rule and was only discovered after the fire of 1917. It was then preserved by the Archaeological Service and converted into an exhibition, featuring several rooms with plenty of artefacts depicting the history of the church before and after the Fire of 1917.