We formulate the fundamentals of the geothermal method for determining the hydrate saturation of bottom sediments. According to laboratory experiments (A.A. Trofimuk Institute of Petroleum Geology and Geophysics, A.V. Nikolaev Institute of Inorganic Chemistry), detecting gas hydrates in bottom sediments requires measurement of thermal conductivity at least twice at one bottom site, using a cylindrical probe with different heater power values. Changing the latter permits controlling gas hydrate stability and instability. A low-power probe does not destroy gas hydrates and permits measuring the true thermal conductivity of the sediments. Increasing heater power destroys gas hydrates near the probe and drastically increases effective thermal conductivity. Comparison between true and effective thermal conductivity clearly shows the presence of gas hydrates in the sample or their absence from it. A technique was proposed for the quantitative interpretation of changes in the temperature field of a cylindrical probe. It permits quite a precise determination of the mass of gas hydrate that decomposed in the layer surrounding the probe over a certain period. Also, it permits a rough estimation of the gas hydrate content of the sediments. Thermal conductivity can be measured in the field with submersible multichannel thermoprobes, which are commonly used for studying the heat flow through the bottom of water basins. Now it is important to perform field experiments, so that we gain the necessary experience with the geothermal method.