The Church of Saint Demetrius is the largest church in Greece and one of the most important Byzantine monuments in the world. A UNESCO World Heritage Site since 1988, it was built in the 4th century on the spot where the patron saint of Thessaloniki was martyred. The five-aisled basilica houses rare 7th and 8th century Byzantine mosaics of outstanding artistic value. The underground crypt contains Roman baths where Saint Demetrius was imprisoned and executed. The church remains an active place of worship with daily services. Visitors can explore both the magnificent nave and the atmospheric crypt museum below