The structural, electronic, and spectroscopic properties of a high-pressure phase of methane hydrate (MH-III) are studied by first principles electronic structure calculations. A detailed analysis of the atomic positions suggests that ionization of hydrogen-bonded water molecules occurs around 40GPa and centering or symmetrization of hydrogen-bonds occurs around 70 GPa. These pressures are much lower compared with ioninzation around 55 GPa and centering around 100 GPa in pure ice. The transition may be observed with low-temperature IR/Raman spectroscopy of OH stretching modes or neutron diffraction. PACS numbers: 62.50.+p,71.15.Pd Methane hydrate (MH), known as Burning Ice, is a special class of ice that contains methane molecules in cages or networks of hydrogen bonded water molecules. Low pressure phase of methane hydrate (MH-I) forms sI structure of cages [1]. MH-I, abundant in the deep ocean, has been attracting attention of the industry as a key material of new energy resource, whose amount is estimated twice as much as the total fossil fuel reserve [1].MH is also known as an important material for understanding the mystery of the atmosphere of Titan, the largest satellite of Saturn. The conventional theory [2] could not explain abundant methane gas in Titan's atmosphere because MH-I inside Titan was assumed to decompose into ice and methane around 1 or 2 GPa, and escape to the atmosphere to be photodecomposed in early stage of Titan's history. To understand this mystery is one of the goals of the Cassini-Huygens spacecraft, which started its journey in 1997 and will arrive at Saturn system in 2004 [3].On the earth, in 2001, Loveday et al. [4,5] discovered new phases of MH by X-ray and neutron diffraction experiments under high pressure: MH-I transforms to MH-II ( sH cage structure) at 1 GPa, and then to MH-III phase (orthorhombic filled ice structure) at 2 GPa, which survives at least up to 10 GPa. Other researchers reported similar high pressure phases [6,7]. Recently Hirai et al. [8] reported that MH-III survives up to 42 GPa at room temperature. Shimizu et al. [9] have measured the site-and pressure-dependence of CH-and OH-vibration frequencies in these phases up to 5.2 GPa. Discovery of these high pressure phases allows us a new explanation of abundant methane gas in Titan's atmosphere: methane gas may be reserved in thick layers of MH-III under Titan's surface and gradually emit to the atmosphere from the reservoir [4]. * Electronic address: tiitaka@riken.jp; URL: http://atlas.riken.go.jp/~iitakaIn this article, we focus on the features of MH-III as a promising material for investigateing centering or symmetrization of hydrogen bonds between water molecules. Advantage of using MH-III for studying the centering is that it is expected to occur at much lower pressure than that of pure ice (ice VII-ice X transition) [10,11,12,13,14,15,16,17,18,19,20], which may make the difficult high-pressure experiments easier. In the following, we calculate the crystal structure and vibrational spectra of MH-III by ...