An apparatus includes heat-generating structure disposed in an environment having an ambient pressure, and a cooling system for removing heat from the heat-generating structure. The cooling system includes a fluid coolant, structure which reduces a pressure of the coolant to a subambient pressure at which the coolant has a boiling temperature less than a temperature of the heat-generating structure; and structure which directs a flow of the liquid coolant at the subambient pressure so that it is brought into thermal communication with the heat-generating structure, the coolant then absorbing heat and changing to a vapor.