Multidisciplinary oceanological investigations of the Haakon Mosby Mud volcano were carried out during cruise 40 of R/V Akademik Mstislav Keldysh in summer, 1998. All major structures of the volcano were described and sampled, composition and distribution patterns of biota were studied using two Mir manned submersibles. Information about the composition of bottom sediments and interstitial water, as well as about hydrocarbons in sediments and in seawater, was also obtained during the expedition.