Description: Gas hydrates were recovered from a mud diapir (Mound 11) located on the southern continental slope off Costa Rica at a water depth of about 1000 m during expedition M54/2 with RV METEOR in September 2002. A massive layer of solid methane gas hydrate was retrieved from the base of a gravity corer at a sediment depth of 2 m. Several mound-shaped and carbonate-covered mud diapirs were previously discovered off Costa Rica and Nicaragua during the ongoing research within the framework of the Sonderforschungsbereich 574 "Volatiles and Fluids in Subduction Zones" but solid hydrates were recovered only from Mound 11. This structure has a diameter of about 300 m and protrudes the surrounding seafloor by about 20 m. The seamount sits on approximately 1 km of hemipelagic sediment disrupted by normal faults, which could provide fluid and mass transport pathways from greater depth to the sea floor. Video observations of Mound 11 revealed the presence of authigenic carbonates and bacterial mats indicating methane-charged fluids reaching the sediment surface. Water sampling and gas chromatographic analyses showed methane enrichments in the overlying bottom water confirming the release of methane. In-situ temperature measurements indicated elevated heat flow within the hydrate-bearing sediment strata. Pore fluids recovered from the mound were strongly depleted in dissolved chloride and enriched in boron indicating a deep origin of rising fluids and gases. The results of the ongoing isotopic analysis of gas hydrates and pore waters will be used to further constrain the source of methane-rich fluids.