When water is supercritical (over 705°F and 218 atmospheres) its properties change dramatically and it is better than liquid water or steam for generating electricity and for electrolysis to produce hydrogen. Also, the greater amount of heat enables cogeneration, i.e. multiple uses of the heat, directly and for the generation of electricity, for greater efficiency. Moreover, geothermal fluid is returned to the reservoir following its uses. Geothermal is a very clean form of renewable energy, using a small surface area and having little effect on the surface ecology.