A hot spring on the ocean floor, where heated fluids exit from cracks in the Earth's crust. Most hydrothermal vents occur along the central axes of mid-oceanic ridges, which are underwater mountain ranges that wind through all of the deep oceans. The best-studied vents are at tectonic spreading centers on the East Pacific Rise and at the Mid-Atlantic Ridge. However, vents are also found over hot spots such as the Hawaiian Islands and Iceland, in back-arc basins such as those in the western Pacific, in shallow geothermal systems such as those off the Kamchatka Peninsula, and on the flanks of some underwater volcanoes and seamounts. Major parts of the mid-oceanic ridges, including those of the Indian Ocean and Southern Hemisphere, are poorly explored, but available evidence suggests that these areas should also have extensive hydrothermal venting. Hydrothermal vent sites, or closely grouped clusters of vent deposits and exit ports, may cover areas from hundreds to thousands of square feet (tens to hundreds of square meters). Individual vent sites may be separated along mid-ocean ridges by more than 1000 mi (1600 km).