The crypt of Boulogne-sur-mer, forming part of the foundations of the Notre-Dame basilica, is a veritable labyrinth of underground rooms. What makes it so impressive is its size: 100 metres long, with a surface area of 1,400 m2, it's one of the biggest crypts in France. Another remarkable thing about it is the painted decoration: from floor to ceiling, the walls and vaults are covered with frescoes. The crypt is also a site of archeaological interest, a testimony of the antiquity of the medieval church and the age-old origins of the city. It holds a treasure-trove of religious art, as well as a collection of precious stones that came from the ancient cathedral.