The many caves in limestone-dominated regions have given rise to various types of blind fauna. The largest of them are crabs, but most are insects, including mosquitoes, butterflies, spiders, locusts, and common flies. Although they are blind, exposure to light is usually fatal to such species. Rivers contain several kinds of freshwater fish, the most plentiful of which are sturgeon, whitefish, and European carp. Mackerel account for the largest percentage of fish taken from the Black Sea. There are no sharks or other dangerous fish in these waters, but a rare Black Sea seal breeds along the rocky coast north of Varna. Mineral Resources The country's mountains contain a variety of metallic and nonmetallic minerals. A few are of good quality, but most of these occur in very small quantities. Iron and coal, which are basic to a metallurgical industry, are mined, but neither of them is of the proper variety or quality nor are they available in adequate quantities to be used economically. Largest deposits of iron ore occur in the far western Stara Planina and the Strandzha mountain range. There are smaller deposits in the vicinity of Burgas, along the Black Sea coast, and near Sofia to the north and west of the city. Estimated reserves total in excess of 10 million tons. Coal has been located in some twenty small deposits. There is an anthracite basin in the Stara Planina twenty miles north of Sofia and another in the extreme northwest end of the range. Bituminous coal occurs in a larger basin in the central Stara Planina, but brown coals and lignite are much more abundant. Copper, lead, and zinc are mined in quantities that exceed domestic requirements. Bulgaria ranks high in the production of them among the eastern and southeastern European countries and exports small amounts of them. Among the other metallic ores, Bulgaria has three of the more important alloying metals--manganese, molybdenum, and chromium--but the manganese is of poor quality. Uranium has been discovered in several deposits near Sofia and is being extracted from one or more of them. Gold occurs in a number of locations but in small quantities. Of the fuels, coal is by far the most abundant and most important to the economy. The search for oil and natural gas resources was intensive in the early post-World War II years, and what were hoped to be valuable fields were discovered in the early 1960s. Production, however, reached a peak in the latter part of that decade. If it becomes economic to exploit them, there are oil-bearing shales west of Sofia and in the northwestern region of the country. The extent of these shales appears to be limited, but their potential is believed to be considerably greater than that of the oil-bearing formations where the crude product is extracted by pumping.