The recent detection of plumes of methane venting into the Martian atmosphere indicates the probable presence of a substantial subsurface hydrocarbon reservoir. Whatever the immediate source of this methane, its production (whether by biogenic or abiogenic process) almost certainly occurred in association with the presence of liquid water in the deep (5+ km [3+ mi]) subsurface, where geothermal heating is thought to be sufficient to raise crustal temperatures above the freezing point of water. Indeed, a geologic evidence that the planet once possessed vast reservoirs of subpermafrost groundwater that may persist to the present day exists. If so, then methane generation has likely spanned a similar period of time, extending over a considerable part of the geologic history of Mars. As on Earth, the venting of natural gas on Mars indicates that substantial amounts of gas are likely present, either dissolved in groundwater or as pockets of pore-filling free gas beneath the depth where the pressure-temperature conditions permit the formation of gas hydrate. Hydrate formation requires the presence of either liquid water or ice. The amount of water on Mars is unknown; however, the present best geologic estimates suggest that the equivalent of a global layer of water 0.5–1 km (0.3–0.6 mi) deep may be stored as ground ice and groundwater beneath the surface. The detection of methane establishes the subsurface of Mars as a hydrocarbon province, at least in the vicinity of the plumes. Hydrocarbon system analysis indicates that methane gas and hydrate deposits may occur in the subsurface to depths ranging from approximately 10 m (30 ft) to 20 km (10 mi). The shallow methane deposits may constitute a critical potential resource that could make Mars an enabling stepping stone for the sustainable exploration of the solar system. They provide the basis for constructing facilities and machines from local Martian resources and for making higher energy-density. chemical rocket fuels for both return journeys to Earth and for more distant exploration.