International Ocean Discovery Program (IODP) Expedition 366 has two primary science objectives.The first objective is devoted to coring a series of sites at the summit and flanks of three large (up to 50 km diameter and 2 km high) serpentinite mud volcanoes in the Mariana forearc (within 100 km west of the Mariana Trench).This objective addresses the broad scientific aim of examining processes of mass transport within the subduction zone of a nonaccretionary convergent margin.In detail, the plan is to recover mudflow materials to (1) examine processes of mass transport and geochemical cycling within the forearc of a nonaccretionary convergent margin; (2) ascertain the spatial variability of slab-related fluids within the forearc environment as a means of tracing dehydration, decarbonation, and water-rock reactions in subduction and suprasubduction zone environments; (3) study the metamorphic and tectonic history of this nonaccretionary forearc region; (4) investigate the physical properties of the subduction zone in relation to dehydration reactions and seismicity; (5) document microbial activity associated with subduction zone material from great depth; and (6) explore linkages among these subduction-related processes, including seismicity, while placing the effects of these processes within a historical context.The second objective establishes long-term seafloor observatory sites by emplacing cased boreholes at summit (conduit) holes in three mud volcanoes (at Expedition 366 proposed Sites MAF-11A, MAF-9B, and MAF-15A) and removing the circulation obviation retrofit kit (CORK) body from Ocean Drilling Program Hole 1200C.These activities set the foundation for future deployments of sensors and samplers with the possibility of deploying a CORK-Lite structure within the boreholes.CORK-Lites provide a framework for conducting temporal observations that will allow one to "take the pulse of subduction" in an active nonaccretionary convergent plate margin and establish a platform for in situ experimentation.