The upper ~500 m of igneous ocean crust is fractured and permeable, harboring the largest hydrologically active aquifer on Earth. We know that microbes inhabit this aquifer, and we also know that microbes are abundant and play active roles in rock alteration of exposed outcrops at the seafloor. We do not know the extent of microbial colonization in the subseafloor, the diversity and activity of this crustal biome, or its role in modulating geochemical exchange between crust and ocean. These fundamental questions will be addressed during Integrated Ocean Drilling Program (IODP) Expedition 336, the primary science objective of which is to investigate the nature of the subseafloor deep biosphere in oceanic crust and overlying sediments.

These investigations will be initiated during Expedition 336 by installing multilevel subseafloor borehole observatories ("CORKs") at three sites (395A, NP-1, and NP-2) for long-term coupled microbiological, biogeochemical, and hydrological experiments. The basaltic crust will also be characterized by coring parts of the crust, by collecting downhole in situ petrophysical data by wireline logging, and by conducting hydrologic (packer) experiments. Coring at four sites will characterize the overlying sediment section.

Operations during Expedition 336 will lay the foundation for long-term monitoring, experimentation, and observations by subsequent remotely operated vehicle (ROV) or submersible dive expeditions. The installed CORKs will be used in perturbation and monitoring points for single- and cross-hole experiments.