In places where the Earth's tectonic plates collide or diverge, volcanoes appear.

Volcanoes can be terrestrial or submarine. There can be submarine volcanoes, which erupt periodically, but also phenomena similar to others that accompany terrestrial volcanoes, like submarine correspondent of the geysers, named hydrothermal vents (hot vents).

The submarine vents, linked to the submarine volcanic activity, trigger an unusual life abundance on the bottom of the ocean. In these areas, animal communities emerge, formed by gigantic species if we compare them to their counterparts at the surface. These animals are usually de-pigmented, white (but not necessarily) and blind, even if some worms and bivalves can be bright red due to the respiratory pigment hemoglobin.

Usually, there we can find clams (up to 30 cm (12 inch) long), mussels, crabs, snails, octopuses, shrimps, tube worms (over 3 m (10 ft) long), echinoderms and even some fish (like tonguefish).

All these communities are very different from most of the Earth's communities; they do not rely on photosynthesis but on chemosynthesis made by bacteria, using energy delivered by the center of the Earth.

These animals are not attracted here by the warm water expelled by the hot vents, but by the abundant food resource. Here, the bacteria are hundreds of times more abundant than in other ocean zones.

The dark, hot water (up to 500o C) gushing through the 10 m (30 ft) long furnaces is loaded with sulfur. Furnaces are formed by the sulfur salt deposits, formed at up to 260 atmospheres pressure encountered at such depths.

The bacteria oxidate the sulfur and hydrogen sulfide to sulfur acid (which forms metallic salts) and the energy they get is used for synthesizing organic compounds, like terrestrial plants do it using sun light, starting from carbon dioxide and water.

The animals here are so adapted to eat these bacteria that some worms do not have guts, and are totally dependent on the symbiosis with these bacteria, having a special bag filled with bacteria, called trophosome.

Clams filter the bacteria, and crabs eat the bacterial mucus deposed over the rocks.

Fish and echinoderms prey on all the smaller species.

In these deep sea communities, some researchers found densities 100 times higher than on surface communities.

Of course, all these animals had to adapt to the extremely toxic acid and sulphuric water and to the high temperatures. Some minute worms can make their tubes on the furnaces, where water can be up to 300° C.

The life oases around the hot vents can spread over 100 m (300 ft) length and 30 m (90 ft) width.

Some volcanic blasts can destroy these communities, but usually they recover quickly.

Some specialists say the hot vents fuel the ocean with salts that ensure the ocean's stable chemical composition, crucial for the maintenance of oceanic life communities.