Chiba borders Ibaraki Prefecture to the north at the Tone River, Tokyo and Saitama Prefecture to the west at the Edo River, the Pacific Ocean to the east and Tokyo Bay around its southern boundary. Most of Chiba lies on the hilly Bōsō Peninsula, a rice farming region: the east coast, known as the Ninety-Nine League Plain, is an especially productive area. The most populous zone, in the northwest of the prefecture, is part of the Kantō region that extends into the urban agglomeration of Tokyo and Saitama. The Kuroshio Current flows near Chiba, which keeps it relatively warm in winter and cooler in summer than neighbouring Tokyo.