Hydrothermal vents are fissures on the seabed that discharge water heated by magma beneath the crust. They form mostly where tectonic plates pull apart along mid-ocean ridges, as well as at volcanic hotspots, and the hottest vents can release fluids above 400 degrees C.
Minerals dissolved in the vent water precipitate when they hit cold seawater, building chimneys such as black smokers. Vents support ecosystems based on chemosynthesis rather than sunlight, including tubeworms, clams, and microbes.
Source: Hydrothermal vent, Wikipedia
Image: Black smoker in the Atlantic Ocean, NOAA, public domain, via Wikimedia Commons.