Hydrothermal vent – an ocean bed site where hot, sulphur‐rich water vents from a geothermal source; with a unique fauna including associated fishes.

Like hot springs and geysers on land, hydrothermal vents form in volcanically active areas, often on mid-ocean ridges, where Earth’s tectonic plates are spreading apart and where magma wells up to the surface or close beneath the seafloor.

Hydrothermal fluid temperatures can reach 400°C (750°F) or more, but they do not boil under the extreme pressure of the deep ocean. As they pour out of a vent, the fluids encounter cold, oxygenated seawater, causing another, more rapid series of chemical reactions to occur. Sulfur and other materials precipitate, or come out of solution, to form metal-rich towers and deposits of minerals on the seafloor. 

Hydrothermal vents transfer heat and chemicals from the Earth's interior and help regulate the chemical composition of the world's oceans. In doing so, they accumulate vast quantities of potentially valuable minerals on the seafloor.