Leg 118 of the Ocean Drilling Project was conducted by the JOIDES Resolution in the southwest Indian Ocean between October and December 1987. By drilling near the Atlantis II Fracture Zone at its intersection with the Southwest Indian Ridge, the shipboard scientific party hoped to penetrate lower crustal or upper mantle material thought to be exposed on the floors and walls of the fracture zone. Although drilling within the Fracture Zone proved impossible, Site 735 B on an adjacent transverse ridge resulted in an unprecedented success. A 500 meter core of intact oceanic Layer 3 was drilled with an 86% recovery, providing a unique opportunity to examine and describe a suite of lower crustal rocks. Samples from the site are being used to describe the magmatic and thermal history, alteration events, extent of seawater penetration and geophysical properties of lower crustal rocks. The information gathered during this cruise will yield valuable data concerning the physical and chemical processes which form and alter the ocean crust. Muehlenbachs and Clayton (1976) have suggested that the ocean's oxygen isotopic value of +0.0 SMOW has not changed appreciably over 200 to 300 million years due to the ocean acting as a buffer. Enrichment and depletions of oxygen isotopes caused by exchanges between seawater and the crustal rocks are balanced. By observing the oxygen isotopic variations in oceanic crust, we hope to see to what extent exchange between seawater and crust has occurred. These exchange and/or replacement processes occur during both low temperature penetration and high temperature hydrothermal circulation of seawater through the rocks. Combined oxygen isotopic and petrologic data obtained from this study and the study of ophiolite sequences help determine the role seawater-crust interactions have played in establishing and maintaining a constant oxygen isotopic value for the ocean. This study has established a downcore oxygen isotopic profile for ODP Site 735B, correlates the penetration of seawater via cracks and veins with the variation of delta (super 18) 0 and determines water/rock ratios for the system using temperatures inferred from the metamorphic assemblages.