Geothermal deep-ocean vents are undersea hot springs that occur in clusters along the mid-ocean ridges. Nutrients and energy supplied by vents support communities of deep-sea organisms found in no other environment.

Most deep-ocean vent action is powered by the heat of the same bodies of magma that drive sea-floor spreading. A vent forms when seawater seeps downward through cracks in the flanks of a mid-ocean ridge to depths of 1.25–2 mi (2–3 km), about halfway through the thickness of the oceanic crust,

Black smoker. Photograph by P. Rona. OAR/National Undersea Research Program (NURP)/National Oceanic and Atmospheric Administration.

and is there heated to 750–840°F (400–450°C). This super-heated water then re-ascends to the center of the mid-ocean ridge and emerges as a fast jet at about 350°C.

As this hot jet mixes with cold (34–36°F [1–2°C]) ocean water, hydrogen sulfide conveyed in solution from the deep...