The Hydrogen Water Cycle is the very beautiful and simple process of water - H2O - splitting into oxygen (which can be released into the air) and hydrogen (which is then stored as an energy source). Hydrogen as an energy source can be used to power internal combustion engines, fuel cells, jet engines, space shuttles and many other machines. When these machines run on hydrogen they produce very little or no pollution. Their main byproduct is water, which can then be split into oxygen and hydrogen again. The cycle is infinitely repeatable.