Certain gases can combine with water to form solids-gas hydrates-that are stable at high pressures and low temperatures1,2. 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 gas3-9. If large volumes of methane are stored in these reservoirs, outgassing may play an important role during climate change10-12. Gas hydrates in oceanic sediments may in fact comprise the Earth's largest fossil-fuel reservoir2,13. But the amount of methane stored in gas-hydrate and free-gas zones is poorly constrained2-9,13-18. 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 (~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.