Coal deposits are scattered in about twenty small deposits. Because of difficult geological conditions, however, only a few of the deposits are exploited. Anthracite is mined in the Svoge basin, located in the Iskur gorge area of the Stara Planina, north of Sofia. Bituminous coal is mined in the same mountain range, in the area between Gabrovo and Sliven. The deposit at Sliven was reported to contain a very small quantity of coking-grade coal--a quantity far below the needs of the iron and steel industry. In addition to large annual imports of coking coal, Bulgaria has also imported from 250,000 to 465,000 tons of coke per year. The main source of brown coal for many years has been the Pernik basin in the upper Struma valley, about nineteen miles southwest of Sofia. In the 1971-75 period brown coal mining is to be substantially expanded at the Bobov Dol deposit in the Rila mountain range, south of the Pernik basin. The Babino mine in the Bobov Dol coalfield is scheduled to become the largest underground coal mine in the Balkans. Reserves in this deposit, however, are equivalent to only about five to six years' production at the 1970 rate of brown coal output. Lignite is mined mainly in the Maritsa basin, near Dimitrovgrad in the Thracian Plain, and in the Sofia Basin. The Maritsa basin, particularly the area known as Maritsa-Iztok (Maritsa-East), has become the basic source of coal production, contributing about 50 percent of the country's output. Aside from planned new mine construction, the Maritsa-Iztok complex is to be rebuilt and modernized. Production problems at this mine have not yet been solved satisfactorily. Coal-bearing strata have not been fully identified; equipment is utilized to only about 40 percent of capacity; and the organization of labor is poor. Substantial improvement also remains to be attained in processing the coal for market, in view of its high ash and moisture content. Unsolved problems also remain in the manufacture of coal briquettes. In the 1971-75 period substantial investment is to be devoted to the expansion and modernization of coal mines. New mines with an annual capacity of about 4 million tons are to be built. Three-fourths of the investment funds are to be concentrated on three major production centers. The relative investment shares of these centers were planned to be: Maritsa-Iztok complex, 41 percent; Bobov Dol complex, 25 percent; and the Georgi Dimitrov mine at Pernik, 10 percent.