This volume covers Leg 20 of the cruises of the Drilling Vessel Glomar Challenger. Leg 20 sailed from Yokohama, Japan to Suva, Fiji, drilling at nine sites from September to November of 1971. Studies of the Pacific Ocean had indicated that tangible evidence for its earliest history was most probably locked in the sediments and crust that now form the abyssal sea floor southeast of Japan and east of the Mariana Trench. The overall geologic history of this exceptionally deep region was the prime focus of Leg 20 of the Deep Sea Drilling Project. The great water depths (19,800+ft.) and the relatively great thicknesses of sediments, particularly interlayered sequences of chalk and chert, provided formidable obstacles to the prime goals. These stringent circumstances called for optimal drilling conditions both with respect to weather and equipment. Although this combination kept us from one of our main goals, the age of the oceanic crust, 13 holes were successfully drilled including 4 in deeper water and with longer drill strings (195B, 196, 197, 199) than ever previously attempted in the ocean. The stratigraphic horizons above the basement provided important new data on the early history of the Pacific Ocean, giving additional evidence on plate motions, equatorial crossings, oceanic sedimentation processes, and showing that the crust in the western Pacific is Jurassic or older.