Iceland’s largest energy company is considering construction of the world’s longest underwater electric cable so the nation can sell its vast geothermal and volcanic energy to the European market. By the end of the year, state-owned energy company, Landsvirkjun, will complete a study of building a sub-sea cable that could deliver as much as five terawatt-hours (5 billion kilowatt-hours) annually to Europe, enough electricity to power 1.25 million homes. Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is energy that determines the temperature of matter. Earth’s geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, from volcanic activity, and from solar energy absorbed at the surface. The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface.