Ocean Drilling Program (ODP) Leg 191 was devoted mainly to two engineering goals, installing a borehole seismometer inside a cased hole and tests of the hard rock reentry system. Those results were achieved, and the borehole seismometer has already yielded useful seismological data showing that the observatory has a low noise profile rivaling quiet land stations. In preparation for emplacing the observatory casing, several holes were drilled at the observatory site (Site 1179), penetrating a 375-m-long sedimentary section and 100 m of igneous crust. Cores from these two sections have been used for a suite of ancillary studies that improve our knowledge of the tectonics and properties of the crust as well as changes in North Pacific climate. Studies of the sedimentary section include investigations of biostratigraphy, anomalous high-carbonate content layers, discrete ash layer chronology and stratigraphy, sediment geochemistry, porosity and permeability, gamma ray logs, and anisotropy of magnetic properties. Among other things, these studies imply that the sedimentary column has a significant terrigenous component, despite the distance from land, and that there were periods when this input fertilized surface waters and stimulated surface productivity. Studies of the igneous basement include investigations of isotopic ratios and paleomagnetism. Nd and Sr isotope ratios suggest that the crust is distinct from normal mid-ocean-ridge basalts, perhaps because of the influence of a nearby plume that may have formed Shatsky Rise. Paleomagnetic data indicate that Site 1179 formed within a few degrees of the equator and has drifted approximately 39 degrees northward. In sum, Leg 191 postcruise science investigations add to the legacy of Pacific geologic data with contributions that will help understand Pacific sediments, igneous crust, and environment.