Doubtful Sound is the deepest (421 metres) and second longest (40 kilometres) of the South Island’s fiords ( called ‘Sound of Silence’). The powerful serenity of the fiord, named Doubtful Harbour by Captain James Cook, who didn’t sail into the inlet because it looked a bit tight, contrasts with the nearby Milford Sound.

It contains two distinct layers of water that don’t mix. The top few meters is fresh water, fed by runoff from the surrounding mountains. Below this is a layer of salt water from the sea. The difference in refractive index between these two layers makes it difficult for light to penetrate. As a result, many deep-sea species – such as black coral – grow in the comparatively shallow depths.

Doubtful Sound has some splendid waterfalls, particularly during the wetter seasons. In the Hall Arm, the Browne Falls cascades 619 metres; Helena Falls at Deep Cove tumbles 220 metres. Wildlife is another reason to visit this fiord – keep an eye out for bottlenose dolphins, fur seals and penguins.