A hydrothermal vent, also known as a Submarine Hydrothermal System, is a jet of water heated by geothermal heat and its fissure vents that erupt from the seabed. It is usually found in areas where volcanic activity occurs frequently, continental plates move, sea basins, and hot spots. Common land types are hot springs, fumaroles, and geysers. Submarine plumes are often formed on the seabed. Compared with other seabed areas at the same depth, organisms are usually more prosperous near hydrothermal vents. They rely on decomposing minerals flowing out of hydrothermal vents for food. Chemosynthetic bacteria and archaea form the bottom of the food chain here, supporting a diversity of life including giant tubeworms, some clams and arthropods. Active hydrothermal vents are also thought to exist on Jupiter's moon Europa, and there may be ancient hydrothermal vents on Mars.

Hydrothermal vents are most typical on mid-ocean ridges, such as the Eastern Pacific Rise and the Mid-Atlantic Ridge, where continental plates separate and new ones are continually formed.

The temperature of the water ejected from hydrothermal vents can be as high as 60 to 464°C, relative to the ambient water at this depth, which is typically about 2°C. At the same time, due to the extremely high hydrostatic pressure at this depth, hydrothermal vents may become supercritical fluids. Its critical point in pure water at 218 standard atmospheric pressure is 375 °C. At a depth of 3,000 meters underwater, the pressure exceeds 300 standard atmospheric pressure (sea water is denser than fresh water), so it becomes a supercritical fluid at 407 °C, with properties between gas and liquid.

Because of the abundance of massive sulfide deposits on the seafloor, hydrothermal vents are an important area for seafloor development. Exploitation of these areas is considered to be of great economic value. The development of these Haitian minerals may damage the ecosystem near the hydrothermal vents, so many protection and control measures are needed before development.