This volume covers Leg 91 of the cruises of the Drilling Vessel Glomar Challenger. Leg 91 was dedicated to the deployment of the Marine Seismic System (MSS) at a site in the Southwest Pacific Basin, approximately 1000 km east of the Tonga Trench. The MSS is a downhole seismometer system developed by the Naval Ocean Research and Development Activity (NORDA) and the Defense Advanced Research Projects Agency (DARPA) to monitor regional seismicity in the deep-ocean environment for potential use in verifying compliance with nuclear test-ban treaties. The MSS deployment strategy was to emplace the BIP in a hole drilled by Glomar Challenger into the basaltic basement that underlies the oceanic sediments, which has several potential advantages over locating seismometers within the sediment column or directly on the seafloor. This volume documents the drilling and seismic experiments conducted by Glomar Challenger at DSDP Site 595, where the MSS was successfully deployed, and the hydraulic piston coring at Site 596, approximately 8 n. mi. west of Site 595. Leg 91 had both engineering and scientific objectives. The former centered on the operational testing of the complete MSS system. Leg 91 was the first full-scale deployment of the MSS into a new borehole. The scientific objectives of seismic experiments were of two principal types: (1) those related to the general problems of assessing and understanding the seismic recording environment of the oceanic crust below the sediments and (2) those pertaining to the specific problems of crustal and upper mantle structure in the southwest Pacific. Glomar Challenger sailed from Wellington, New Zealand, on January 16, 1983 to begin Leg 91, and arrived in Papeete, Tahiti on 20 February, 1983 to end the leg.