Heavy water (deuterium oxide, 2H2O, D2O) is a form of water that contains only deuterium (2H or D, also known as heavy hydrogen) rather than the common hydrogen-1 isotope (1H or H, also called protium) that makes up most of the hydrogen in normal water. The presence of the heavier hydrogen isotope gives the water different nuclear properties, and the increase in mass gives it slightly different physical and chemical properties when compared to normal water.

Hydroelectricity, or hydroelectric power, is electricity produced from hydropower. In 2020 hydropower generated one sixth of the world's electricity, almost 4500 TWh, which was more than all other renewables combined and also more than nuclear power.Hydropower can supply large amounts of low-carbon electricity on demand, making it key to many secure and clean electricity grids. With a dam and reservoir it is also a flexible source of electricity, since the amount produced by the station can be varied up or down in seconds or minutes to adapt to changing energy demands. Once a hydroelectric complex is constructed, the project produces no direct waste, and it almost always has a considerably lower output level of greenhouse gases than fossil fuel powered energy plants. However, when constructed in lowland rainforest areas, where inundation of a part of the forest is necessary, they can emit substantial amounts of greenhouse gases.

The construction of a hydroelectric complex can cause significant environmental impact, principally in loss of arable land and population displacement. They also disrupt the natural ecology of the river involved, affecting habitats and ecosystems, and the siltation and erosion patterns. While dams can ameliorate the risks of flooding, they also contain a risk of dam failure, which can be catastrophic.