Undersea geysers that run along the ocean ridge that were first discovered by deep submersible vehicles(or DSVs - including one called Alvin, in 1977) who were bunking off the search for lost submarines. These ridges are like giant mountain ranges (the largest on earth) 4,000-8,000 m under the sea. They can spew superhot (400°C) seawater filled with minerals into the cold ocean. This sometimes creates the chimneys that make the vents so visible, as the minerals precipitate out to form rock.

Studying these structures involved dragging one up from the seafloor to the surface. A tricky escapade, since it involved breaking off the chimney at the base and winching it carefully upwards, without fracturing it (I watched this on TV, in case you were wondering). Some vent chimneys can be kilometers high, yet they are hollow, delicate structures.

A source of much interest to science since they seem to be oases of life in the otherwise deserted plains of the sea floor. Best of all, the temperature gradient means that it's a sort of natural chemical factory. The movement of substances from hot to cold and back could provide the energy for organic synthesis. Especially interesting because many archaea live in vents, and were first recognised there. Hot temperatures and high mineral (iron, sulphides, calcium) concentrations mean unusual living conditions for the vent inhabitants. These are not restricted to unicellular life, but include worms, crabs and so on that live in cooler waters slightly away from the vent mouth. All these creatures rely on the source of chemical energy provided by the bacteria, since the vents are too far from the sun to provide photosynthetic life

One of many questions that remains is how the creatures that depend on vents move from one to another. Since they only last for a couple of years, it seems strange that the organisms manage to find and colonize (as a group, or singly) a new vent.

Sources: various pages found on Google, and documentaries I remember seeing. Oh, OK! the ONR (office of Naval Research - www.orn.navy.mil) and, randomly, the Graduate College of Marine Studies at the University of Delaware (www.ocean.udel.edu).