The Crypt, is accessed through an opening on the rear of the building, down two short flights of stairs cut in the ‘600 into the wall.
There was a second entrance, now walled up, the left aisle of the church, which is currently visible (but not open to visitors, as located in a private home) in the courtyard at no. 11.
Little more than 11 meters long and 6 m wide, the crypt is oriented as all pre-Romanesque and Romanesque churches and is divided into three naves by four cylindrical columns of stone and four brick pillars, of later date.
The wall that closes on the west side, towards the cloister, the four brick columns and part of the time the above have a brick masonry of mediocre bill late eighteenth century.
Probably, in ancient crypt it continued for another three bays, as in other similar crypts (S. Maria di Cavour and Testona, for example).
All the rest of the internal perimeter wall is brick mixed with river pebble and the floor now presents cobblestones.
The four stone columns, one of which is white veined marble, could come, like pieces of recovery, from Roman buildings from nearby Lomello.