The Basilique Saint-Seurin is one of the most ancient sacred sites in Bordeaux — there has been a place of Christian worship here since at least the 4th century, and the building itself has layers stretching from Merovingian foundations to Romanesque nave to later additions. In 1998 it was inscribed as a UNESCO World Heritage Site as part of the Routes of Santiago de Compostela in France.
The main reason to visit, apart from the architecture, is the crypt. Beneath the nave is a 6th-century underground burial chamber filled with dozens of early Christian sarcophagi — some dating to the 4th and 5th centuries — in an atmospheric and little-visited space. Ask at the entrance to descend.