Water

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.

Appearances in the Halo universe

 * 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 and almost all of Halo 3's levels, in some form or another (excluding water vapor in a latter form). A hydro-processing plant was one of the multiplayer levels in Halo: Combat Evolved.
 * In Halo 2, the entire level Regret takes place on, around, and even underneath at one point, a large lake.
 * 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, are encountered in Halo 3, with minor references to it in Halo 3: ODST.

Trivia

 * Water is utilized as a death barrier in some maps. In Halo 2 maps Headlong and Waterworks, 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, bodies and objects will float on water for a while before sinking.