"Under the choir lies a vast and sombre crypt, the oldest
part of the church. Dating back to carolingian times, it
underwent several transformations. The original flat roof
was relpaced after the great fire of 1161 by arched vaults
and the floor lowered. Here were kept, during Vézelay's
great times, the relics of Ste Mary Magdelene in a tomb
now under the southern transept."

from a Vézelay guide book