Studies indicate that fluids are present in the Earth's crust from the surface to at least 10 to 15 km. Fluids play a vital role in linking various physical and chemical processes by transporting energy and solutes at a wide range of spatial and temporal scales. Below the sea floor, fluids are involved in seismogenic zone dynamics, global chemical cycles, gas hydrate formation, mid-oceanic hydrothermal systems, biological community development, and sediment diagenetic processes (Figure 1). Fluid fluxes through the sea floor and oceanland interfaces play a major role in the global water cycle, but these fluxes remain to be quantified. While the main scientific question centers on the role of fluid in transporting heat and mass in various settings, the following specific issues form the foundation for a better understanding of the role of fluids in subsea floor processes: