Life giving substance generally found today on Mars in the form of ice at its caps and as vapor in the sky. Large amounts of it could be stored underground in areas outside the polar regions, such as under the craters at Sinus Sabaeus. Salty water, which has a lower freezing temperature, could occasionally flow on the surface today. Parts of Mars' interior may be warm enough to allow liquid water to exist. It may come up in a mud volcano. Nasa's current mantra is "follow the water", which its spacecraft do to try to find evidence of past and present habitability.