The Korea Strait is a sea passage in East Asia between Korea and Japan, connecting the East China Sea, the Yellow Sea and the Sea of Japan in the northwest Pacific Ocean. The strait is split by the Tsushima Island into the Western Channel and the Tsushima Strait or Eastern Channel. It is economically important, as many shipping lanes pass through the strait, and both Japan and Korea permit free passage through it. In ancient times, both Buddhism and Mongol invaders passed over the strait to reach Japan. More recently, it was the site of the Battle of Korea Strait in the Korean War. A tunnel running underneath the strait connecting Japan and Korea, the Japan–Korea Undersea Tunnel, has been proposed to connect the two countries. Geography To the north, the Korea Strait is bounded by the southern coast of the Korean Peninsula. And to the south, it by the southwestern Japanese islands of Kyūshū and Honshū. It is about wide and averages about 90 to 100 meters (300 ft) deep. Tsushima Island divides the Korea Strait into the western channel and the Tsushima Strait. The western channel is deeper (up to 227 meters) and narrower than the Tsushima Strait. Currents A branch of the Kuroshio Current passes through the strait. Its warm branch is sometimes called the Tsushima Current. Originating along the Japanese islands this current passes through the Sea of Japan then divides along either shore of Sakhalin Island, eventually flowing into the northern Pacific Ocean via the strait north of Hokkaidō and into the Sea of Okhotsk north of Sakhalin Island near Vladivostok. The water-mass characteristics vary widely because of the low-salinity waters of the southeast coasts of Korea and China. Economic significance Numerous international shipping lanes pass through the strait, including those carrying much of the traffic bound for the ports of southern South Korea. Both South Korea and Japan have restricted their territorial claims in the strait to from shore, so as to permit free passage through it. Passenger ferries travel numerous routes across the strait. Commercial ferries run from South Korean Busan, Geoje to Japanese ports including Fukuoka, Tsushima, Shimonoseki, and Hiroshima. Ferries also connect Tsushima Island with Fukuoka, and South Korea's Jeju Island with the Korean mainland. Ferries connecting Busan and Japanese cities with ports in China also traverse the strait. Japan's territorial waters extend to three nautical miles (5.6 km) into the