Hydrothermal area is the place where hydrothermal fluids elute from seafloor, like hot springs that can be found near volcanoes. Ambient seawater penetrates into the deep subseafloor and chemically reacts with high temperature rocks. By the reaction, hydrogen sulfide is produced from sulfate ion. The seawater loses magnesium ion, and enriched in metal ions such as ferric, copper and manganese ions. Through these processes, the water becomes hydrothermal fluid that has totally different composition from ambient seawater. The hydrothermal fluid, then, goes up from deep subseafloor to seafloor. Hydrothermal fluids don’t come to a boil in spite of very high temperature owing to very high water pressure at deep sea floor, and blow out at above 300oC. The vent that elutes the hydrothermal fluid is called hydrothermal vent. Minerals included in the hydrothermal fluid precipitate around the vent and a structural object like chimney is formed. The object is called a hydrothermal chimney. When the hydrothermal fluid reacts with ambient seawater, sulfides form black precipitates. The fluid looks like black smoke going up from a chimney. Thus, the hydrothermal vent blowing out black hydrothermal fluid is called black smoker. On the other hand, the temperature of hydrothermal fluid becomes lower when it is mixed with low temperature seawater subseafloor. The low temperature hydrothermal water remains clear and colorless when it springs into the sea. The hydrothermal vent blowing out the transparent fluid is called clear smoker.