The islands of Japan are the projecting summits of a huge chain of mountains originally a part of the continent of Asia, from which they were detached in the Cenozoic era. The long and narrow main island, Honshu, measures less than 322 km (200 mi) at its greatest breadth; no part of Japan is more than 161 km (100 mi) from the sea. The coastline of Japan is exceedingly long in proportion to the area of the islands and totals, with the many bays and indentations, about 29,750 km (about 18,500 mi). The greatest amount of indentation is on the Pacific coast, the result of the erosive action of the tides and severe coastal storms. The W coast of Kyushu, on the East China Sea, is the most irregular portion of the Japanese coast. Few navigable inlets are found on the E coast above Tokyo, but S of Tokyo Bay are many of the best bays and harbors in Japan. Between Honshu, Shikoku, and Kyushu is the Inland Sea, dotted with islands and connected with the Pacific Ocean and the Sea of Japan by three narrow straits through which oceanic storms rarely pass. The W coast of the islands of Japan, on the almost tideless Sea of Japan, is relatively straight and measures less than 4830 km (less than 3000 mi); the only conspicuous indentations in the coastline are Wakasa and Toyama bays in Honshu.

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