High amounts of gas were present in sediments cored during Leg 175. The possible origins of these sedimentary gases are (1) production as a result of microbial degradation of sediment organic matter, (2) in situ thermal decomposition of sediment organic matter, and (3) thermal degradation of deeply buried organic matter and migration into shallower sediments. The elevated hydrostatic/lithologic pressures and low temperatures that exist beneath the seabed typically keep gases dissolved in the interstitial fluids of most sediment sequences. In some sediments, gas is sufficiently abundant to exist in the free state or as gas hydrate. During core recovery, gases exsolve from solution as a result of the release of pressure and from warming of the sediment. Gas bubbles and gas expansion pockets were common features of cores recovered during Leg 175.