Doubtful Sound is one of 14 major fjords that were formed 15,000 years ago in a scenic part of New Zealand’s South Island. Some 25 miles (40 km) long and opening onto the Tasman Sea, it is surrounded by steep hills from which hundreds of small waterfalls descend during the rainy season. Its name originated in 1770 during the first voyage to New Zealand by the English explorer Captain James Cook (1728–79). He called the fjord Doubtful Harbor because he was skeptical of being able to sail out again if he entered it. Doubtful Sound is the second-longest and the deepest of the New Zealand fjords, with a maximum depth of 1,380 ft (421 m). It receives fresh water from a hydroelectric power station at its head and from a huge 236 in (6,000 mm) of rainfall annually. Like all fjords, it contains fresh water in its top few yards and a much denser, colder, saltier layer below. There is little mixing between the two. Doubtful Sound is home to bottlenose dolphins, New Zealand fur seals, and many species of fish, starfish, sponges, and sea anemones.