Brought down to earth, nuclear fusion – a process fueled primarily by lithium and deuterium (an isotope of hydrogen), both of which are plentiful in seawater and in the earth’s crust – could provide a major source of low-carbon energy. A fusion power station would use only around 450 kilograms of fuel annually, cause no atmospheric pollution, and carry no risk of accidents that could lead to radioactive contamination of the environment.