undercroft

from The Collaborative International Dictionary of English v.0.48

Undercroft \Un"der*croft\, n. [Under + Prov. E. croft a vault;
   cf. OD. krochte crypt, and E. crypt.] (Arch.)
   A subterranean room of any kind; esp., one under a church
   (see {Crypt}), or one used as a chapel or for any sacred
   purpose.
   [1913 Webster]