[1] At 40 kilometres (25 mi) long, Doubtful Sound / Patea is the second longest, and with a depth of up to 421 metres (1,381 ft) the deepest of the South Island's fiords.

Doubtful Sound was named 'Doubtful Harbour' in 1770 by Captain Cook, who did not enter the inlet as he was uncertain whether it was navigable under sail.

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.

[5] The officers of the expedition, which included Felipe Bauzá y Cañas, a cartographer,[6] also made the first chart of the entrance and lower parts of the Sound, naming features of it.

[7] Following the passage of the Ngāi Tahu Claims Settlement Act 1998, the name of the fiord was officially altered to Doubtful Sound / Patea.

[8] Doubtful Sound lies deep within the Fiordland National Park, about 50 kilometres (31 mi) from the nearest inhabited place, the small town of Manapouri, and is surrounded by mountainous terrain with peaks typically reaching 1,300–1,600 metres (4,300–5,200 ft).

Charles John Lyttelton, 10th Viscount Cobham, Governor-General of New Zealand (1957–1962) wrote about this part of Fiordland: Doubtful Sound (like many of the fiords in the area) is unusual in that it contains two distinct layers of water that scarcely mix.

This layer receives fresh water fed from the high inflows from the surrounding mountains, and stained brown with tannins from the forest.

The catchment basin of Doubtful Sound is generally steep terrain that is heavily forested except for locations where surface rock exposures are extensive.

[21] Recently (2000s), there has been growing concern that the population is in significant decline, with calf survival rates having halved, as well as being noticeably lower than in captivity or in other New Zealand environments.

The reasons for this are unclear, though increased tourism and the fresh-water discharge from the Manapouri Power Station (see Hydroelectricity section below) are considered potential causes.

Deep Cove (45°27′S 167°09′E / 45.450°S 167.150°E / -45.450; 167.150), the innermost end of Doubtful Sound, is the site of discharge of water from the Manapouri Power Station's tailrace tunnels.

The discharge of clear fresh water has affected fauna and flora by letting light into the lower layers of the sound.