Saracen's Head hydrothermal vent, or black smoker, on the Mid-Atlantic Ridge. Black smokers are geologically active seabed vents where magma (molten rock) near the surface heats water in the rock to extremely high temperatures, well over 100 degrees Celsius. It is prevented from boiling by the immense pressure at that depth: this vent is some 3100 metres below the surface. The hot water dissolves a large quantity of mineral salts. When the superheated water meets the very cold surrounding water, the minerals rapidly precipitate out, giving the hot water its characteristic smoky appearance. The minerals form multicoloured crusts around the vent, causing them to grow into tall chimneys over time.