Undercroft n. : A subterranean room of any kind; esp., one under a church (see Crypt), or one used as a chapel or for any sacred purpose.

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Taxonomy
undercroft, London, free encyclopedia, Scotland, ground level, storage room, History, stores, England, Norwich, Norfolk, Articles with unsourced statements, search, cinema, cellar, brick, modern usage, ground, street-level.

Defination from the web:
undercroft rooms at Myres Castle in Scotland circa 1300 were used as the medieval kitchen and a range of stores. ... The undercroft beneath the Houses of Parliament in London was rented out to the conspirators behind the Gunpowder Plot. ...