Hydropower is energy obtained from the gravitational potential energy and kinetic energy of liquid water mass, which is then used for various purposes. Hydropower is the most significant renewable energy source that is also economically competitive with fossil fuels and nuclear energy. Hydroelectric power plants are energy plants where the hydroelectric potential is converted into electricity. The environmental impact of hydroelectric power plants is drastically less than that of traditional energy sources. By generating electricity from hydropower instead of coal, in 2017 the world prevented approximately four billion tonnes of greenhouse gases and avoided a 10% rise in global emissions from fossil fuels and industry. It also avoided 148 million tonnes of air polluting particulates, 62 million tonnes of sulphur dioxide, and eight million tonnes of nitrogen oxide from being emitted. Each method of electricity production has advantages and disadvantages, as well as significantly different environmental effects. Hydroelectric power plants have great potential because of the possibility of producing electricity with the least environmental impact. In order to determine the advantages and disadvantages of building a plant, energy efficiency, environmental impact and the level of greenhouse gas emissions, it is important to take into account the whole "life cycle" of the project; exploration, planning, construction, manufacturing facility, maintenance, renovation and eventually decay.