
Crypte archéologique de l'île de la Cité · Île de la Cité (4th arrondissement)
In-situ remains spanning two millennia: a Gallo-Roman quay, baths and 4th-century ramparts, medieval cellars and church foundations, and Haussmann-era sewers under Notre-Dame.
Philippe Alès, Wikimedia Commons (CC BY-SA 3.0)