Deep Seismic Investigation of the Ontong Java Plateau
 The Ontong Java Plateau (OJP; see Figure 1a), in the western equatorial Pacific Ocean, is one of the largest oceanic plateaus on Earth, covering 1.86 million square kilometers—equivalent to roughly one third of the contiguous United States [Coffin and Eldholm, 1994]. This voluminous plateau is thought to have formed about 120 million years ago and to have caused significant global environmental changes; however, a consensus has not been reached on how the OJP formed. Competing theories include formation through a mantle plume, a meteor impact, and fast spreading ridges. Detailed knowledge of the OJP's crustal and upper mantle structure is necessary to help distinguish among existing models and to develop new models of how the OJP formed. Previous seismic studies acquired data by exploding charges at the ocean's surface and, through seismic analyses of travel times, deducing horizons of reflections and refractions below the ocean floor. However, most of these studies used relatively low energy shots and sparse source and receiver intervals and thus were unable to unambiguously determine the depth of the crust-mantle boundary.
We thank Captain Hitoshi Tanaka and the crew of the R/V Kairei for their skilled work during the cruise. We are grateful to the governments of the Independent State of Papua New Guinea and the Federated States of Micronesia for permission to work in their exclusive economic zones. The cruise represents one of Japan's contributions on the occasion of the fiftieth anniversary of the Intergovernmental Oceanographic Commission (IOC).