Methane hydrate is formed by the compression of underground methane gas and was the most important form of energy in the twenty first century. However, due to a string of explosions in the methane hydrate reserves, as much as 85% of humanity were wiped out in the ensuing ecological disaster that followed. Although not stated in the series, the explosion probably also caused the sky to be covered by clouds which is central to the survival of proxies as further explained later on in the series. It is implied that immediately after this catastrophe happened, humans began the construction of Boomerang Star as part of the the Boomerang Project as well as the comencement of the Proxy Project.