largest island, Jeju-do, lies off the southwest corner of the peninsula and has a land area of 1,825 square kilometers. Other important islands include Ulleung and Liancourt Rocks in the Sea of Japan and Ganghwa Island at the mouth of the Han River. Although the eastern coastline of South Korea is generally unindented, the southern and western coasts are jagged and irregular. The difference is caused by the fact that the eastern coast is gradually rising, while the southern and western coasts are subsiding. Topography and drainage Early European visitors to Korea remarked that the land resembled "a sea in a heavy gale" because of the large number of successive mountain ranges that crisscross the peninsula. The highest mountains are in North Korea. The highest mountain peak in South Korea is Hallasan (), which is the cone of a volcanic formation constituting Jeju Island. There are two major mountain ranges within South Korea: the Taebaek Mountains, and the Sobaek Mountains. Unlike Japan or the northern provinces of China, the Korean Peninsula is geologically stable. There are no active volcanoes (aside from Baekdu Mountain on the border between North Korea and China, most recently active in 1903), and there have been no strong earthquakes. Historical records, however, describe volcanic activity on Mount Halla during the Goryeo Dynasty (no having kissits lowlands are the product of mountain erosion. Approximately 30 percent of the area of South Korea consists of lowlands, with the rest consisting of uplands and mountains. The great majority of the lowland area lies along the coasts, particularly the west coast, and along the major rivers. The most important lowlands are the Han River plain around Seoul, the Pyeongtaek coastal plain southwest of Seoul, the Geum River basin, the Nakdong River basin, and the Yeongsan River and the Honam plains in the southwest. A narrow littoral plain extends along the east coast. A recent global remote sensing analysis suggested that there were 1,833 km2 of tidal flats in South Korea, making it the 17th ranking country in terms of how much tidal flat occurs there. The Nakdong is South Korea's longest river (). The Han River, which flows through Seoul, is long, and the Geum River is long. Other major rivers include the Imjin, which flows through both North Korea and South Korea and forms an estuary with the Han River; the Bukhan, a tributary of the Han that also