hydrothermal vent

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fissure on the floor of a sea from which flows mineral-rich water that has been heated by underlying magma, forming a precipitate of minerals and providing an ecosystem for extremophiles.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a hot spring, on the floor of the ocean, mostly along the central axes of the mid-ocean ridges, where heated fluids emerge from fissures in the Earth's crust