Doubtful Sound is a fiord of superlatives. It is the deepest in Fiordland and measures around 420 m in depth. The full distance from Deep Cove, its eastern end, to the ocean amounts to around 40 km. In its centre, the fiord branches off into five waterways: The First Arm, Crooked Arm, Hall Arm and its Deep Cove in the south; the Thompson and Bradshaw Sound in the north. Towards the open sea, the fiord splits into several islands including Secretary and Bauza Island as well as the Shelter Islands.
First European seaman to discover the fiord was Captain James Cook himself in 1770 naming it the "Doubtful Harbour". In fact, him and his crew had doubted to be able to sail out of its bay again. Next to arrive was Alessandro Malaspina di Mulazzo in 1793, an Italian nobleman working for the Spanish colonial power, who designed the fiord's first maps. Doubtful Sound is sometimes referred to the name "Patea" which may be of Spanish origin since parts of the fiord carry Spanish titles already. Malaspina's cartography guided whalers and sealers, that would arrive within the following decades.
Similar to the rest of Fiordland, Doubtful Sound experiences heavy rainfall throughout the year causing a rich vegetation. High waterfalls such as the Helena at Deep Cove or the Browne Falls (600 m) are very common and here also form a covering layer on top the saline water. The fiord is therefore home to species such as crayfish, black corals, starfish, sea anemones, fur seals as well as a resident group of bottlenose dolphins. Whale species like southern right whales, humpbacks, minke and sperm whales, giant beaked whales, orcas and long-finned pilot whales are regular visitors.