The Basse Oeuvre — "the lower work" — is the ghost of the cathedral that stood before the current one. Tucked against the west end of the Gothic choir, it is a 10th-century Carolingian church, built around 987, that was never demolished when the ambitious Gothic rebuilding began. Instead, it was left standing as a temporary western wall for the new cathedral while construction continued, and then simply forgotten. Today it stands as one of the oldest surviving church buildings in northern France: a plain, low structure of small stones with round-headed windows, its scale dwarfed by the Gothic stonework behind it.
Entry is typically free and combined with a visit to the cathedral. The contrast between the two buildings — separated by a thousand years of architectural ambition — is more eloquent than any guidebook explanation of Gothic construction could be.