The Barents Sea is relatively shallow, lying north of Europe and south of the islands of Svalbard and Franz Joseph Land. To the east, Novaya Zemlya is an extension of the Ural Mountains, which mark the geographical boundary between Europe and Asia. Large areas along the mainland and around the islands are continental shelf of less than 660 ft (200 m) deep. Warm water from the North Atlantic Drift flows in from the southwest, keeping most of the sea ice-free in summer. The Russian port of Murmansk remains free of ice even in the winter. The warm, salty Atlantic water meets cold, less saline Arctic water, and warm, moderately salty coastal water, producing an area of high biological productivity. The spring bloom of phytoplankton starts near the ice edge, where fresh water from melting ice produces a stable surface layer. The phytoplankton form the basis of a food chain that supports a rich fishery, and cod is the most important catch. During the Cold War, Russia maintained a large northern fleet of warships and submarines. Many of these vessels now lie deteriorating in naval ports along the Kola Peninsula, raising fears of possible damage to the marine environment. Particular concerns have been raised about contamination from the nuclear reactors of abandoned submarines.