Hydrographic casts down to the bottom along two zonal sections at 12°N and 13°N (from 144°E to 127°E) were made with a CTD. Their analysis verified the existence of cold and saline abyssal water between the Mariana Ridge and the Kyushu-Palau Ridge. This result provides evidence of flow into the Philippine Sea through the deep gap called the Yap-Mariana Junction. The properties of deep water are variable in the West Mariana basin but quite homogeneous in the Philippine Basin, indicating the transitional nature in the West Mariana Basin and the existence of older bottom water in the Philippine Basin. A close examination suggests that the bottom water is slightly colder in the western part of the Philippine Basin than in the eastern part of the basin. This slightly colder deep water with a hundred kilometer scale in the western Philippine Basin might be related to a broad western boundary current flowing equatorward along the eastern rise of the Philippine Trench.
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