"Scientists have discovered a 'smoking' volcano 3,000 metres below the surface of the Indian Ocean.
The team on board the research vessel RRS Charles Darwin made the find when they detected a huge, dark plume of water, 600 metres thick and over 30 kilometres wide, rising hundreds of metes above a lava-strewn valley on the Carlsberg ocean ridge.
'Black smokers', often teeming with exotic lifeforms, are known to exist in the Pacific and Atlantic Oceans but their discovery in the Indian Ocean is very recent.
EDIT
As convection pushes sections of the Earth's crust apart, molten material rises to fill the gaps. Water percolates down below the seabed and is superheated before gushing out from hot springs or vents at about 300 to 400 Celsius."
EDIT
http://news.bbc.co.uk/2/hi/science/nature/3128457.stm