methane hydrate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A crystalline structure consisting of ice that contains interstitial methane, found in ocean floor sediments and detected on some moons of the outer planets of the solar system.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A gas hydrate that contains methane as the guest molecule; It occurs both in deep sedimentary structures, and as outcrops on the ocean floor.