Certain gases can combine with water to form solids-gas hydrates-that are stable at high pressures and low temperatures(1,2), Conditions appropriate for gas-hydrate formation exist in many marine sediments where there is a supply of methane, Seismic reflection profiles across continental margins indicate the frequent occurrence of gas hydrate within the upper few hundred metres of sea-floor sediments, overlying deeper zones containing bubbles of free gas(3-9), If large volumes of methane are stored in these reservoirs, outgassing may play an important role during climate change(10-12), Gas hydrates in oceanic sediments may in fact comprise the Earth’s largest fossil-fuel reservoir(2,13). But the amount of methane stored in gas-hydrate and free-gas zones is poorly constrained(2-9,13-18). Here we report the direct measurement of in situ methane abundances stored as gas hydrate and free gas in a sediment sequence from the Blake ridge, western Atlantic Ocean, Our results indicate the presence of substantial quantities of methane (similar to 15 GT of carbon) stored as solid gas hydrate, with an equivalent or greater amount occurring as bubbles of free gas in the sediments below the hydrate zone.