Evaporites are crystalline sedimentary rocks that form by the evaporation of natural brines. Having complex mineralogy, there are many varieties of evaporites; the most economically important minerals include: calcite, gypsum, anhydrite, halite, polyhalite, sylvite, carnallite, kainite, kieserite and anhydrite. Although evaporite deposits formed in ancient marine basins are extensive on land, many of these also extend beneath the sea, not only beneath the continental shelves but also under some marginal ocean basins. Present day occurrences of subsea anhydrite, potash and magnesium evaporite deposits have been described along the margins of the Mediterranean Sea as well as in the North Sea and West Baltic Sea.