Haakon Mosby Mud Volcano (HMMV) functions as a spot emitter for methane in the sub-arctic ocean. Fluidised mud associated with large amounts of methane is expelled at the volcano?s centre (eye) ascending from its deep interior. At its surface HMMV is characterised by three key types of sedimentary environments, more or less concentrically arrayed around the eye.Large populations of small tube worms (pogonophora) are distributed along the volcano?s outer rim whereas the sediment next to the eye is covered by mats of sulfur bacteria. The later depend on the H2S released as a product of sulfate reduction by methane oxidizing microbial communities. The central area of HMMV consists of a fine grained grayish sediment uncovered by bacteria. In combination with bottom water data, our pore water investigations suggest the bacteria-covered sediments to be very efficient biofilters, prohibiting diffusive methane discharge into the bottom water. Thus, beside channelised methane release, the volcano?s eye is recognized to be the ultimate zone of methane emission.Partly ROV-guided in situ experiments and sampling were performed during 3 expeditions with R/V ?L?Atalante?/ROV "VICTOR 6000" in 2001, R/V ?Polarstern? (ARK XVIII/1b) in 2002, and R/V ?Polarstern?/ ROV "VICTOR 6000" (ARK XIX/3b) in 2003, respectively.A special bottom water sampler was deployed for bottom water sampling and bottom currents have been measured by ROV-operated current meters.