The Romanesque construction built by Bishop Fulbert comprised a lower church, a vast 105-metre-long nave without a transept surmounted by a roof structure. The cathedral today gives us a clear indication of the size of the Romanesque basilica because the width of the new construction was dictated by that of the Romanesque building.
What survives of the Romanesque basilica is the crypt, which dates from the 1020s. The Gothic cathedral was built over this crypt. Because it was designed to channel the attending pilgrims, it has a remarkable shape, consisting of a wide corridor that curves round into a hemicycle at its eastern end to direct the flow of pilgrims around the place of sanctity. Three deep chapels lead off this ring. The crypt is distinctive in that it does not have a central room. It is a wide corridor designed to accommodate the long processions of pilgrims who came to pray at the relic of the Virgin and the effigy of Our Lady of the Underground.