Deep sea vents are hydrothermal fissures which are present under the sea waters on ocean beds. These vents openings on the ocean beds formed due to the activity that is continuously happening inside the earth. These are locally very common at volcanically active places where tectonic plates are moving apart, ocean basins and hotspots.
The most famous deep sea vents are probably the ones which are present in the Yellowstone National Park. These vents throw out superheated saline water which is not even suitable for any life forms. Also, these vents are located at depths where sunlight does not reach so the ecosystem that exists here is surviving completely without any sunlight. Bacteria are formed here which form the food of the slightly bigger organisms. Blind crabs and shrimps also exist at such levels.
With the superheated water that is thrown out of the vents, also minerals are thrown out which give the water a black colour. This makes the emission look like a smoke arising from the vent. The water is at a temperature of around 400 degrees and when it comes in contact with sea water at 2 degrees, the minerals solidify and settle down thereby increasing the height of the sediments around the vent. These vents form a chimney like structure with these sediments up to a height of 60 metres or even more in some cases.