Unlike many other seas, most of the bottom of the Norwegian Sea is not part of a continental shelf. It is at a great depth of about two kilometres on average. Rich deposits of oil and natural gas are found under the sea bottom and are being explored commercially. The coastal zones are rich in fish that visit the Norwegian Sea from the North Atlantic for spawning. The warm North Atlantic Current gives stable and high water temperatures, so that unlike the Arctic seas, the Norwegian Sea is ice-free throughout the year.