Doubtful Sound (like many of the fiords in the area) is unusual in that it contains two distinct layers of water that scarcely mix. The surface layer , referred to as the Low Salinity Layer or LSL, consists of diluted seawater with a typical salinity value of less than 10 psu. This layer receives fresh water fed from the high inflows from the surrounding mountains, and stained brown with tannins from the forest. The outflow of freshwater from the tailrace discharge from the Manapouri Power Station also influences the thickness of the LSL. Below the LSL is a layer of warmer, undiluted seawater with a salinity value of around 35 psu. The dark tannins in the fresh water layer make it difficult for light to penetrate. Thus, many deep-sea species grow in the comparatively shallow depths of the Sound. Such species include black coral "Antipathes fiordensis", which is normally found at depths of but can be found at just in Doubtful Sound and is within the range of qualified recreational divers.