We present results of experimental studies on the formation of gas hydrates (clathrates) at conditions of geophysical interest. Clathrate hydrates formed by a reaction of gas at ice Ih surfaces are always found to be mesoporous to macroporous with pores sizes between 100 to 400 nm and pore volumes of approximately 25--40% for CH4, Ar and N2 hydrate, and smaller pores of 20 to 100 nm with a porosity of approximately 10--20% for CO2 hydrate. The three-dimensional sponge-like microstructure occurs in single crystalline grains of typically a few &mgr;m size and was observed by field-emission scanning electron microscopy. It forms over a wide range of p-T conditions below the ice Ih melting. The porous microstructure is stable for at least several months, even close to the clathrate decomposition, and is proposed to be formed by local differences in the energy balance between hydrate formation and ice decomposition. The results presented are considered of potential major importance for the understanding of the behavior of natural gas hydrates found e.g. in polar ice sheets and permafrost regions, and also in some celestial bodies. ¿ 2000 American Geophysical Union