Marine occurrences of gas hydrate are normally confined to the top few hundred meters of sediments along deep continental margins. The zone of stability for gas hydrate is limited in depth by increases in temperature below the seafloor. We use thermodynamic calculations to show that gas hydrate can exist in a metastable state below the usual base of the stability zone. We estimate that gas hydrate can be overheated by several degrees and that it may persist in this metastable state in the seafloor for as long as 106 years. Sudden decomposition of metastable hydrate should produce substantial pore pressure in the sediments, contributing to slope failure in locations where gas hydrate is found. Such a mechanism might help to explain why slumping appears to be more frequent than average during the interval around the last glacial maximum.