We present a study of the structure of a fully deuterated methane hydrate under the geological conditions found in the world's oceans. In situ high-resolution neutron diffraction experiments have been performed at temperatures of 220, 275, and 280 K and a pressure of 100 bar, corresponding to the conditions at 1000 m water depth. The data were analyzed with a combination of Rietveld refinement and maximum entropy methods. From the Rietveld refinement, precise atomic parameters of the host lattice could be determined, indicating increasing distortions of the structure of the cages at elevated temperatures and pressures. Debye–Waller factors of the encaged CD₄ molecules have been found to exceed the values of the Debye–Waller factors of the D₂O molecules considerably. In the large cage of structure type I the thermal center-of-mass displacements of the guests are 5–10 times larger than those of the water molecules. From the maximum entropy analysis maps of the scattering length density have been obtained, showing details of the vibrational amplitudes of the atoms in methane hydrate. The Debye–Waller factors of all molecules have been found to deviate considerably from a simple spherical geometry. ; NRC Pub: yes