Alternative energy sources are being explored as fossil fuels diminish, including solar, geothermal, and energy from water and wind. Solar energy can be collected passively through building design or actively through solar collectors. Geothermal energy harnesses heat from the earth's interior by drilling wells into hot underground water or steam. Hydroelectric power generates electricity from the force of running water through dams and turbines, while tidal power uses the daily movement of ocean tides. Wind farms employ propeller wind turbines that can power entire communities in windy areas.