Doubtful Sound is a very large and naturally imposing fjord (despite it's name) in Fiordland, in the far south west of New Zealand. It is located relatively close to the smaller but more famous and accessible Milford Sound. It took second place after said Fiord as New Zealand's most famous tourism destination.
Doubtful Sound was named 'Doubtful Harbour' by Captain Cook, who did not enter the inlet as he was uncertain whether it was navigable under sail. It was later renamed Doubtful Sound by whalers and sealers.
A Spanish scientific expedition commanded by Alessandro Malaspina visited Doubtful Sound in February 1793 to conduct experiments measuring the force of gravity using a pendulum, a part of the effort to establish a new metric system. The officers of the expedition, which included Felipe Bauzá y Cañas, a cartographer, also made the first chart of the entrance and lower parts of the Sound, naming features of it. Today these form a unique cluster of the only Spanish names on the map of New Zealand: Febrero Point, Bauza Island and the Nee Islets, Pendulo Reach and Malaspina Reach.
There are three distinct arms to the sound, which is the site of several large waterfalls, notably Helena Falls at Deep Cove, and the Browne Falls which have a fall of over 600 metres. The steep hills are known for their hundreds of waterfalls during the rainy season.
Access to the sound is either by sea, or by the Wilmot Pass road from the Manapouri Power Station. Most areas of the sound itself are only accessible by sea however, as the road network in this area of New Zealand is sparse or nonexistent, as is the human population.
Doubtful Sound is unusual in that it contains two distinct layers of water that do not mix. The top few meters is fresh water, fed from the high inflows from the surrounding mountains. Below this is a layer of cold, heavy, saline water from the sea. The difference in the refractive index between these two layers makes it difficult for light to penetrate. Thus, many deep-sea species will grow in the comparatively shallow depths of the Sound.
This fiord is home to one of the southernmost population of bottlenose dolphins. The Doubtful Sound bottlenoses have formed a very insular sub-group of the only about 70 individuals, with none having been observed to leave or enter the Sound during a multi-year monitoring regime. Their social grouping is thus extremely close, which is also partly attributed to the difficult and unusual features of their habitat, which is much colder than for other bottlenose groups and is also overlaid by the freshwater layer.
Other wildlife to be found in Doubtful Sound includes fur seals and penguins (Fiordland crested and blue),or even rare large whales (Southern Right, Humpback, Minke, Sperm and some Beaked Whales). The waters of Doubtful Sound are also home to an abundance of sea creatures, including many species of fish, starfish, sea anemones and corals. It is perhaps best known for its black coral trees which occur in unusually shallow water for what is normally a deep water species.
Deep Cove an arm of Doubtful Sound, is the site of discharge of water from the Manapouri Power Station's tailrace tunnels. Deep Cove, like the rest of Fiordland is a unique and mostly pristine environment. The discharge has had an impact, although this is an area naturally high in fresh water inflows (7.6 metres of rain annually).