The international oceanographic community carried out a coordinated series of cooperative cruises and dives with Japanese, Russian and U.S. submersibles between 1993 and 1995 to perform baseline measurements and to establish a seafloor observatory to monitor the active sulfide mound in the TAG hydrothermal field before, during and after drilling by Ocean Drilling Program Leg 158 in October--November 1994. The results of that effort and related work are reported herein as group of 23 papers comprising a special section in this issue of ''Geophysical Research Letters''. ¿ American Geophysical Union 1996