seabed methane clathrates, also known as "flammable ice." Deposits of compressed and frozen methane are fairly common in northern waters. British Columbia has looked at exploiting them.
The gas field is about 50km away from Japan's main island, in the Nankai Trough.
Researchers say it could provide an alternative energy source for Japan which imports all its energy needs.
Offshore deposits present a potentially enormous source of methane but
also some environmental concern, because the underwater geology
containing them is unstable in many places.
A Japanese study estimated that at least 1.1tn cubic metres of methane hydrate exist in offshore deposits.
This is the equivalent of more than a decade of Japan's gas consumption.