Hydrothermals are ephemeral deep oceanic vents formed at mid-oceanic ridges (undersea mountain ranges of seabed creation, where lava rising from the Earth's mantle cools and forms two diverging tectonic plates which move outwards away from each other) in areas of submarine volcanoes. They are oceanic fumaroles (from the Latin 'fumus' = smoke) which emit steam and gas. Solfatara (Italian 'solfo' = sulphur) emit sulphurous gas. Hydrothermal vents can be white smokers or black (sulphurous) smokers.
The liquid which comes out of these vents is a supercritical fluid which exists only when there is no boundary between its liquid and gas phases. At below 2,243 m depth (418 atmospheres of pressure) water does not boil even at the temperature of 400 degrees centigrade at which it emerges from the vents. This homogeneous supercritical fluid diffuses through solids like a gas and dissolves materials like a liquid.
