The Integrated Ocean Drilling Program (IODP) Expedition 337 was the first expedition dedicated
to subseafloor microbiology that used riser-drilling technology with the drilling vessel Chikyu. The drilling Site
C0020 is located in a forearc basin formed by the subduction of the Pacific Plate off the Shimokita Peninsula,
Japan, at a water depth of 1180 m. Primary scientific objectives during Expedition 337 were to study the relationship
between the deep microbial biosphere and a series of 2 km deep subseafloor coalbeds and to explore
the limits of life in the deepest horizons ever probed by scientific ocean drilling. To address these scientific
objectives, we penetrated a 2.466 km deep sedimentary sequence with a series of lignite layers buried around
2 km below the seafloor. The cored sediments, as well as cuttings and logging data, showed a record of dynamically
changing depositional environments in the former forearc basin off the Shimokita Peninsula during the late
Oligocene and Miocene, ranging from warm-temperate coastal backswamps to a cool water continental shelf.
The occurrence of small microbial populations and their methanogenic activity were confirmed down to the bottom
of the hole by microbiological and biogeochemical analyses. The factors controlling the size and viability
of ultra-deep microbial communities in those warm sedimentary habitats could be the increase in demand of
energy and water expended on the enzymatic repair of biomolecules as a function of the burial depth. Expedition
337 provided a test ground for the use of riser-drilling technology to address geobiological and biogeochemical
objectives and was therefore a crucial step toward the next phase of deep scientific ocean drilling.