Gas hydrates are natural gas reservoirs in ice-like crystalline solids, and are stable in pore spaces of submarine sediments in water depths greater than about 300–500 m. They have been recovered in many of the world's oceans, both at larger sub-bottom depths (up to 450 m) by drilling and near the seafloor in shallow cores by gravity-coring. In the latter case, the gas hydrates are related to the sites of enhanced seepage such as cold seeps and mud volcanoes [Ginsburg and Soloviev, 1998].

Multidisciplinary field investigations during the two cruises have revealed new, large hydrate-bearing seepage structures in the Sea of Okhotsk, a northwestern marginal sea of the Pacific Ocean (Figure l). The Derugin Basin at the central part of the Sea of Okhotsk, the zone of intensive gas seepage and hydrate accumulation, was studied during two cruises of the R/V Akademik M.A. Lavrentyev (LV) of the Russian Academy of Sciences (RAS), in August and October 2003 within the framework of the CHAOS project (hydroCarbon Hydrate Accumulations in the Okhotsk Sea) supported by funding agencies in five nations.