The crypt beneath Chartres Cathedral is the largest in France, stretching 220 metres from end to end. It was built in the 11th century on foundations that go back to a much earlier Carolingian church, and below that to a Gallo-Roman site — tradition has it that a sacred druidic well existed here long before any Christian structure. The puits des Saints-Forts (Well of the Strong Saints), a 35-metre shaft sunk into the rock, is one of the crypt's strangest features; early martyrs were reportedly thrown into it.
Visits to the crypt are by guided tour only, departing from the south portal of the cathedral. The tour includes chapels and sections not accessible independently, and gives a sense of how the cathedral above sits on centuries of continuous sacred use stretching back before Christianity.