Leg 139 of the Ocean Drilling Program (ODP), completed on September 11, was the first part of a proposed two-leg program to investigate hydrothermal processes and products at seafloor-spreading centers covered with sediment. Sedimented spreading centers are unique and provide ideal targets for drilling; a regionally continuous, relatively impermeable sediment cover over zero-age crust limits the recharge and discharge of hydrothermal fluids and thermally insulates the underlying crust. This creates a thermal regime in which high formation temperatures can occur at comparatively shallow levels in the crust. Where discharge of fluid does occur, very large hydrothermal sulfide deposits can be produced. The sediment may also preserve a relatively continuous stratigraphic record of magmatic, tectonic, and thermal events, providing clues to the spatial and temporal variability of these processes. The sediment also stabilizes the drill-string and allows holes to be “spudded” without the operational expense of a complicated seafloor guidebase.