The Philippine Sea stretches east to west between the Marianas Islands and the Philippines, and from north to south between Japan and Palau. This warm sea is swept by the North Equatorial Current, which turns north to form the Kuroshio Current. The water becomes very warm in the summer, and the area is a breeding ground for typhoons. The Philippine Sea is underlain by the Philippine Plate, an oceanic plate that is subducting at the Philippine and Ryukyu trenches. The plate is split into two main basins. The westernmost Philippine Basin is the deepest and oldest, separated from the West Mariana Basin by the Kyushu–Palau Ridge. This ridge, and the Iwo-Jima and West Mariana ridges, are the remnants of island arcs associated with ancient subduction zones.