undercroft
Noun
Meaning
A subterranean room of any kind; esp., one under a church (see Crypt), or one used as a chapel or for any sacred purpose.
Source: Webster's dictionarySynonyms
Antonyms
Examples
Some of the lower parts of the monastic dormitory, an extension of the South Transept, survive in the Norman undercroft of the Great School, including a door said to come from the previous Saxon abbey.
The noise turned out to be the then-tenant's widow, who was clearing out the undercroft directly beneath the House of Lords—the room where the plotters eventually stored the gunpowder.
The only traces of St. Dunstan's monastery remaining are round arches and massive supporting columns of the undercroft and the Pyx Chamber.