Water

There is more information available on this subject at Water on the English Wikipedia.

The molecular structure of water, note the angle of the bonds, which greatly increase the uses of this chemical.

Water, chemical formula H₂O, is a common chemical substance that is essential to all known forms of life. In typical usage, water refers only to its liquid form or state, but the substance also has a solid state; ice, and a gaseous state; water vapor or steam. Its chemical make-up is a covalent bond of two hydrogen atoms and a single oxygen. The covalent bond is strong, but the energy released from breaking this bond is low and hard to capture. It is a polar molecule, forming weak hydrogen bonds between the positive hydrogen ends and the negative oxygen end. Due to its polar characteristics, water cannot mix with oil or other non-polar substances.

The Global Water Campaign and its GWC Trans-African Pipelines were designed to transport the still relatively clean melt water from Mount Kilimanjaro across the entire African continent. Multiple facilities of the GWC, including A2 Hydroelectric Facility and Voi Municipal Water Pump House 03, were sites of combat during the Battle of Earth.^[1]

Behind the scenes

Water is seen in plain sight during the majority of all the Halo games. It appears in six of the ten levels of Halo: Combat Evolved, nine of Halo 2's levels, in three Halo 3 levels, and in seven maps in Halo: Reach.
In Halo 2, the entire level Regret takes place on, around, and even underneath at one point, a large lake.
A hydro-processing plant was one of the multiplayer levels in Halo: Combat Evolved.
Water is utilized as a death barrier in some maps and the player is killed upon hitting the water or when hitting the water becomes inevitable.
Frank O'Connor is a self-confessed fan of video game "water".
In Halo 3 and Halo: Reach, bodies and objects will float on water for a while before sinking.