About 71 % of the earth's surface is covered in water, and we can harness some of it to create energy. The two most common water energy technologies are hydropower and tidal power. Hydropower is electrical energy derived from falling or running water. The movement of the water turns the blades of a turbine, which is connected to a generator, which produces electricity. Hydropower is the most commonly used form of renewable energy. Tidal power converts the natural rise and fall of the tides into electricity using variety of technologies including tidal fences, garages and turbines. Additionally, wave power can be harnessed to capture energy from waves on the surface of the ocean using a special buoy. Energy from water is considered a renewable energy resource because it uses the Earth's water cycle and gravitational pull to generate electricity. It also does not directly emit greenhouse gas emissions or air pollutants. However, depending on the water energy technology, negative environmental land-use impacts can be an issue. Water is also heavily used in energy production processes. For example, water is used in power generation to generate steam, cool nuclear thermal plants, extract and process fossil fuels, and to irrigate biofuel crops.