Ice at Bottom of Ocean?

[DanBlather]

Can anyone explain this to me:

Trapped within sheets of ice up to 500 meters (1,640 feet) thick is an estimated 40 trillion cubic feet of crystalline methane encased in an ocean trench called the Nankai Trough, 30 miles (50 kilometers) off the coast of the main Honshu Island.

from here.
Do they mean water ice is trapping the crystalline methane?

[Squink]

Methane forms a structure I hydrate with two dodecahedral (20 vertices thus 20 water molecules) and six tetradecahedral (24 water molecules) water cages per unit cell.

[LSLGuy]

never mind