This paper presents the requirements and baseline design for a subsurface Life Support System (LSS) to support an underground habitat. The purpose of this effort was to demonstrate/validate the feasibility of building an operational habitat to support survivable and enduring operations of a deeply based ICBM weapon system. Described is an overall life support design for a crew of 100 to 600 persons that encompasses all required life support subsystems, arrangement and construction of Habitat enclosures and protection from the effects of a subsurface environment.Effects of habitat layout, type of power source, environment, rock temperature and moisture content, crew size and mission length were investigated. Regenerative and non-regenerative systems were compared on the basis of life cycle cost.Results of this Life Support System study were much different than those previously conducted for space and submarine application due to the difficulty in rejecting heat. Spacecraft can use radiation, submarines convection, whereas an underground habitat must reject heat via conduction to the surrounding rock or a phase change heat sink.The project culminated in the construction and operation of a pre-prototype, manned Habitat. This manned test confirmed the performance of the integrated subsystems and provided a data base with which to design and build a prototype underground Habitat.