The Korean Peninsula is surrounded by water on three sides, and is connected to China to the north. As South Korea is located at the gateway to the continent, it has a long history of active trad- ing through the ocean. Korean navigation tech- niques involving tidal currents, ocean currents, and wind are also much more advanced than what is widely known. In terms of topography and cli- mate, Korean seas have more complicated struc- tures than those of any other country. Not only do they have various physical, chemical, and geo- logical factors—such as seasonal ocean currents, tides, and complex topographical features—they have a range of marine lives with unique charac- teristics.

The eastern coastline of Korea is relatively sim- ple, while the south and west coasts are complex with diverse coastal landforms. There are a total of 3,418 islands located around the Korean coast- al areas, mainly concentrated in the South Sea and

the Yellow Sea.The Yellow Sea, which extends into the East

China Sea, is characterized by shallow water less than 80 m deep and gently sloping submarine topography. It is named after its yellowish color, which arises from sediment-rich river water from the Yangtze and the Yellow River of the Chinese mainland. It has a low average salinity (30 – 33‰) that can be attributed to the influence of freshwater from the many rivers owing from the Korean Peninsula and China. Sea surface salinity is further reduced during the rainy season of the summer due to an increase of freshwater from the land. During the warmer conditions of spring to summer, thermocline and halocline develop at a depth of 20 – 30 m, leading to strati cation with high temperatures at the surface and low tempera- tures at the bottom. In winter, the cooling of the surface water causes vigorous vertical mixing, thus making strati cation disappear. The Yellow

Sea has a well-developed tidal current that arises from strong tidal activities.

The southern coast is mainly composed of the Korea Strait, which separates Korea and Japan. It is characterized by shallow water with an average depth of less than 100 m, and is a narrow water- way through which the Tsushima Current passes. Throughout the seasons, it is greatly affected by the fluctuation of the Kuroshio Current, which carries subtropical seawater characterized by high temperatures and high salinity. During the summer, seawater with low salinity from the East China Sea is distributed in the upper layer, while high-salinity seawater from the Kuroshio Current (known as Kuroshio Intermediate Water) spreads in the lower layer. Only Kuroshio Intermediate Water remains during the winter, due to the ab- sence of low-salinity seawater from the East Chi- na Sea.

The east coast has a relatively simple coastline

with deep water that reaches depths of 2,000 – 3,000 m. It has well-developed narrow beaches, but the steep slopes of coastal and seabed topog- raphy prevent the development of tidal ats and continental shelves. The submarine topography of the east coast is divided into three major basins. The Japanese Basin, which is over 3,000 m deep, is located to the north. In the south, the Yamato Uplift stands in the center, with the Ulleung Basin to the west and the Yamato Basin to the east. The Tsushima Current – which flows into the East Sea through the Korean Strait – spreads along the surface of the East Sea, while the East Sea Proper Water can be seen in the deep sea layer. With a salinity of 34.0 – 34.1% and a temperature of 0 – 1°C, the East Sea Proper Water displays char- acteristics that are only witnessed in the East Sea. The Tsushima Current (flowing from the south) and the Lehman Cold Current ( owing from the north) also meet at the East Sea.