Extraterrestrial liquid water
The Gliese 581 planetary system contains multiple planets that may be candidates for surface water, including Gliese 581c, Gliese 581d, might be warm enough for oceans if a greenhouse effect was operating, and Gliese 581e.
Liquid hydrogen
Liquid hydrogen is a common liquid rocket fuel for rocketry applications — both NASA and the United States Air Force operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons). In most rocket engines fueled by liquid hydrogen, it first cools the nozzle and other parts before being mixed with the oxidizer — usually liquid oxygen (LOX) — and burned to produce water with traces of ozone and hydrogen peroxide. Practical H–O rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust, which can actually increase specific impulse, despite the incomplete combustion.
Liquid hydrogen can be used as the fuel for an internal combustion engine or fuel cell. Various submarines (Type 212 submarine, Type 214 submarine) and concept hydrogen vehicles have been built using this form of hydrogen (see DeepC, BMW H2R). Due to its similarity, builders can sometimes modify and share equipment with systems designed for liquefied natural gas (LNG). However, because of the lower volumetric energy, the hydrogen volumes needed for combustion are large. Unless direct injection is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses.