Definitions for "Undercroft"
A subterranean room of any kind; esp., one under a church (see Crypt), or one used as a chapel or for any sacred purpose.
A crypt i.e. a vaulted underground room beneath a church which may be used either as a burial place or for storage
In Christian church architecture: A vaulted space, sometimes underground, below the main level of a church or chapel.
Keywords:  souls, lower, building, level
The lower level of the All Souls building.