Ocean Drilling Program Leg 201 was the first ocean drilling expedition dedicated to the study of life deep beneath the seafloor. Its seven sites were selected to represent the general range of subsurface environments that exist in marine sediments throughout most of the world's oceans. In water depths as great as 5300 m and as shallow as 150 m, the expedition drilled as deep as 420 m into oceanic sediments and the underlying rocky crust. The sediments ranged in temperature from 1° to 25°C and in age from 0 to almost 40 Ma.

To document metabolic interactions in deeply buried marine sediments and the differences between those interactions in ocean-margin sediments and in open-ocean sediments, shipboard scientists measured an unprecedented array of metabolic reactants and products in the interstitial waters of Leg 201 sediments. To document subsurface communities, shipboard scientists initiatied an unprecedented number and range of micobial experiments. To document the nature of subsurface environments, shipboard scientists documented an exhaustive range of core and downhole geophysical and sedimentological properties.

Active microbial respiration occurs throughout the sediment column at every site. Subseafloor respiration is supported at all sites by the diffusion of sulfate down from the overlying ocean, as well as by the dissolution of iron- and manganese-bearing minerals. At the open Pacific sites, respiration deep beneath the seafloor is also supported by the transport of sulfate, nitrate, and oxygen from water circulating through the underlying basaltic crust. At both the open Pacific sites and the Peru margin sites, electron-accepting pathways often described as competitive consistently co-occur beneath the seafloor. Iron reduction and manganese reduction often co-occur with sulfate reduction and methanogenesis. Sulfate reduction and methanogenesis co-occur everywhere. In all of the sedimentary environments sampled during Leg 201, sedimentary properties and, by inference, past oceanographic conditions affect current rates of microbial activities.

Microbial abundances are much higher in sediments buried on the continental shelf of Peru than in sediments of the open Pacific Ocean. The open Pacific sites contained some of the lowest average microbe concentrations ever observed in deep-sea sediments. In contrast, some of the sediments recovered on the Peru shelf contained the highest concentrations of microbes ever observed beneath the seafloor. At the Peru shelf sites, the concentration of sedimentary microbes was highest in a narrowly focused zone of anaerobic methanotrophy tens of meters beneath the seafloor.

The recovered sediments and fluids will be studied further to document the composition of subseafloor communities, the principal controls on rates of subsurface activity, the influence of past oceanographic conditions on current activity in deeply buried sediments, and the effects of subseafloor biogeochemical processes on Earth's surface world.