The Probert Encyclopaedia of Architecture

UNDERCROFT

In architecture an undercroft is a subterranean room of any kind but especially one under a church, or one used as a chapel or for any sacred purpose.
Research Undercroft

UNDERPINNING

In architecture underpinning refers to the material and construction used for the support of a building introduced beneath a wall which has already been constructed - the substitution of a new foundation for an old one. The term is also applied to the foundation, especially of a frame house.
Research Underpinning