Abstract: A thermal liner in a diving suit has a layer of incompressible phase change materials for storing latent heat energy and for later releasing the std energy while changing phase. This thermal liner provides thermal protection for divers' wetsuits, drysuits, and hot-water suits using stored energy from phase change materials, for extreme cold water diving. The thermal liner can function as an emergency backup heat source upon power failure when electrically-heated drysuits are used, or as an emergency backup heat source in case of interruption of warm water supply when hot-water diving suits are used. It can also be used as a supplemental source of heat for divers wearing passively-insulated wetsuits or drysuits to prolong acceptable durations in cold water missions. The thermal liner gives divers an emergency "come home" capability in case of power failure within drysuits supplied with an electrically-heated undergarment, or of an interruption of the warm water supply to a hot-water suit.