Description: Shallow gas hydrate accumulation in mud volcanoes in the Costa Rica forearc was postulated before, but is now proven by a find in surface sediments at the southwestern slope of the recently discovered Mound 11, a mud volcano located 30 km arcward from the trench, on the continental slope off Costa Rica at 1000 m water depth. The gas hydrate content of the recovered core was up to 60% and consisted mainly of methane hydrate. The δ13C (−45.2‰ to −43.3‰ PDB) and δD (−125‰ to −143‰ SMOW) values of methane from sampled hydrates indicate a deep (thermogenic) source of fossil methane generated by degradation of organic matter within the subducted slab. Near surface faults and deeply cutting faults, identified in multichannel seismic reflection profiles, provide pathways for fluid migration through the ∼6 km thick margin wedge into the ∼1 km of overlying terrigenous sediments. Mound 11 overlies a bottom simulating reflection at 340 m bsf and transport of sediment and methane-rich fluids from greater depth through the gas hydrate stability zone is suggested. The upper core segment (0–150 cm bsf) is composed of mud breccia and fluid channels, which indicates mud expulsion from Mound 11. Anaerobic methane oxidation is indicated by sulfate and methane depletion, hydrogen sulfide formation and an increase of alkalinity in the interface between the upper sediment unit and the lower laminated sediment unit where the gas hydrate is interbedded. The seawater-like sulfate and chloride concentrations and the concave up chloride profile measured in pore water of the upper core unit may rather reflect seawater influx than fluid outflow at this sampling site. The inflow is possibly driven by (episodic) mud and fluid discharge in the center of the mud mound creating shallow convective circulation cells.