The Iheya North Hydrothermal Field is located in the middle Okinawa Trough. During the Integrated Ocean Drilling Program (IODP) Expedition 331, the Deep Hot Biosphere Project, five sites (C0013~C0017) were drilled in the Iheya North Hydrothermal System from September 1 to October 4, 2010 in order to investigate metabolically diverse subseafloor microbial ecosystems and their physical and chemical settings. Site C0016 located in the active hydrothermal vent site and sulfide-sulfate mound at North Big Chimney (NBC) has the recovery rate of only 4.7%, but the core included the first Kuroko-type, sphalerite-rich black ore ever recovered from the modern seafloor. The other four sites yielded interbedded hemipelagic and volcaniclastic sediment and volcanogenic breccias and pumice that are variably hydrothermally altered and mineralized, in the zeolite to greenschist facies. Detailed temperature profiles at Sites C0014 and C0017 display irregularities suggestive of lateral flow.The analysis of interstitial water and headspace gas yielded complex patterns with depth and laterally at most sites over distances of only a few meters. Documented processes include formation of brines and vapor-rich fluids by phase separation and segregation, uptake of Mg and Na by alteration minerals in exchange for Ca, leaching of K at high temperature and uptake at low temperature, anhydrite precipitation, microbial oxidation of organic matter and anaerobic oxidation of methane utilizing sulfate, microbial methanogenesis. Shipboard analysis has not confirmed the presence of an active deep hot biosphere. Cell abundances are much lower than those found in previous Ocean Drilling Program/ IODP sites on continental margins, and attempts at culturing were generally unsuccessful.