A new high-pressure phase of methane hydrate (HPMH) has been identified on the basis of its high optical relief, distinct pressure-temperature phase relations, and Raman spectra. In-situ optical observations were made in a hydrothermal diamond-anvil cell. The new high-pressure phase may be critical for understanding the stable and metastable phase relations amongst the hydrates beneath the seafloor and permafrost. It is also important for understanding the behavior of water and gases at high pressure in the planetary bodies like Europa in the outer solar system.