Where
Under water volcanos happen where two plates meet under the sea.
For example the ring of fire and Iceland.

What
They are volcanos under the water.

Chimneys and life around
Chimneys are under water volcanos they create spires of rock.
They are some times called “black smokers”.
Bacteria feeds on the chemicals coming out of the chimney.
And the bacteria supports other life forms like shrimps.
It can create ecosystems in the deep sea that would normally require sun light.

Why doesn’t the sea evaporate?
Some of the sea does evaporate but is always replaced by rivers and rain so the ocean soaks up the heat.

Atolls
Is the brim of a volcano and then coral grows on the brim.
An atoll only happens in the tropics because coral only grows in the warm seas.