Methane (CH 4) concentrations and CH 4 stable carbon isotopic composition (d 13 C CH 4) were investigated in the water column within Jaco Scar. It is one of several scars formed by massive slides resulting from the subduction of seamounts offshore Costa Rica, a process that can open up structural and stratigraphical pathways for migrating CH 4 . The release of large amounts of CH 4 into the adjacent water column was discovered at the outcropping lowermost sedimentary sequence of the hanging wall in the northwest corner of Jaco Scar, where concentrations reached up to 1,500 nmol L -1 . There CH 4 -rich fluids seeping from the sedimentary sequence stimulate both growth and activity of a dense chemosynthetic community. Additional point sources supplying CH 4 at lower concentrations were identified in density layers above and below the main plume from light carbon isotope ratios. The injected CH 4 is most likely a mixture of microbial and thermogenic CH 4 as suggested by d 13 C CH 4 values between -50 and -62 % Vienna Pee Dee Belemnite. This CH 4 spreads along iso-pycnal surfaces throughout the whole area of the scar, and the concentrations decrease due to mixing with ocean water and microbial oxidation. The supply of CH 4 appears to be persistent as repeatedly high CH 4 concentrations were found within the scar over 6 years. The maximum CH 4 concentration and average excess CH 4 concentration at Jaco Scar indicate that CH 4 seepage from scars might be as significant as seepage from other tectonic structures in the marine realm. Hence, taking into account the global abundance of scars, such structures might constitute a substantial, hitherto unconsidered contribution to natural CH 4 sources at the seafloor.