Milford Sound is named after Milford Haven in Wales, while the Cleddau River which flows into the sound is also named for its Welsh namesake. The Māori named the sound Piopiotahi after the thrush-like piopio bird, now extinct. Piopiotahi means "a single piopio", harking back to the legend of Māui trying to win immortality for mankind—when Maui died in the attempt, a piopio was said to have flown here in mourning.
Milford Sound runs 15 kilometres inland from the Tasman Sea at Dale Point (also named after a location close to Milford Haven in Wales)—the mouth of the fiord—and is surrounded by sheer rock faces that rise 1,200 metres (3,900 ft) or more on either side. Among the peaks are The Elephant at 1,517 metres (4,977 ft), said to resemble an elephant's head, and The Lion, 1,302 metres (4,272 ft), in the shape of a crouching lion.
Milford Sound sports two permanent waterfalls, Lady Bowen Falls and Stirling Falls. After heavy rain temporary waterfalls can be seen running down the steep sided rock faces that line the fiord. They are fed by rain water drenched moss and will last a few days at most once the rain stops.
With a mean annual rainfall of 6,412 mm (252 in) each year, a high level even for the West Coast, Milford Sound is known as the wettest inhabited place in New Zealand and one of the wettest in the world. Rainfall can reach 250 mm (10 in) during a span of 24 hours. The rainfall creates dozens of temporary waterfalls (as well as a number of major, more permanent ones) cascading down the cliff faces, some reaching a thousand metres in length. Smaller falls from such heights may never reach the bottom of the sound, drifting away in the wind.
Accumulated rainwater can, at times, cause portions of the rain forest to lose their grip on the sheer cliff faces, resulting in tree avalanches into the sound. The regrowth of the rain forest after these avalanches can be seen in several locations along the sound.
Seals, penguins, and southernmost population of bottlenose dolphins frequent the waters, and whales especially humpback whales and southern right whales are increasingly observed due to recoveries of each species. The sound has been identified as an Important Bird Area by BirdLife International because it is a breeding site for Fiordland penguins.
Milford Sound was initially overlooked by European explorers, because its narrow entry did not appear to lead into such large interior bays. Sailing ship captains such as James Cook, who bypassed Milford Sound on his journeys for just this reason, also feared venturing too close to the steep mountainsides, afraid that wind conditions would prevent escape.
The fiord was a playground for local Maori who had acquired a large amount of local marine knowledge including tidal patterns and fish feeding patterns over generations prior to European arrival. The fiord remained undiscovered by Europeans until Captain John Grono discovered it c.1812 and named it Milford Haven after his homeland in Wales. Captain John Lort Stokes later renamed Milford Haven as Milford Sound. Following the passage of the Ngai Tahu Claims Settlement Act 1998, the name of the fiord was officially altered to Milford Sound / Piopiotahi.
While Fiordland as such remained one of the least-explored areas of New Zealand up to the 20th century, Milford Sound's natural beauty soon attracted national and international renown, and led to the discovery of the Mackinnon Pass in 1888, soon to become a part of the new Milford Track, an early walking tourism trail. In the same year, the low watershed saddle between the Hollyford River and the Cleddau River was discovered, where the Homer Tunnel was to be developed about sixty years later to provide road access.
As of the 2006 census, just 120 people lived in Milford Sound, most of them working in tourism or conservation.
Milford Sound attracts between 550,000 and 1 million visitors per year. This makes the sound one of New Zealand's most-visited tourist spots even with its remote location and long journey times from the nearest population centres. Many tourists take one of the boat tours which usually last one to two hours. They are offered by several companies, departing from the Milford Sound Visitors' Centre.
Tramping, canoeing, and some other water sports are possible. A small number of companies also provide overnight boat trips. There is otherwise only limited accommodation at the sound, and only a very small percentage of tourists stay more than the day. Tourists usually stay in Te Anau or Queenstown.
The Milford Discovery Centre & Underwater Observatory is located in Harrisons Cove on the north side of the fiord. Situated within the Piopiotahi Marine Reserve, the underwater observatory allows visitors to view the fiord's unique marine environment at a depth of 10 metres. Due to a natural phenomenon called 'deep water emergence,' deep-water animals such as black coral can be viewed in the shallow waters surrounding the observatory. A dark surface layer of fresh water, stained brown by tannins from the surrounding forest, along with cold water temperatures allow the black corals to grow close to the surface throughout Milford Sound and Fiordland.
On rainy and stormy days tourists can admire the play of the wind with the numerous waterfalls in Milford Sound. When meeting the cliff face the powerful wind often goes upward and waterfalls with a vertical drop get caught by wind, causing the water to go upwards.
Milford Sound is also a scenic cruise destination for cruise ships.
By road, Milford Sound is 307 km from Queenstown and 279 km from Invercargill (about four hours' drive), with most of the tour buses to the sound departing from Queenstown. Some tourists also arrive from the smaller tourism centre of Te Anau, 121 km away. There are also scenic flights by light aircraft and helicopter tours to and from Milford Sound Airport. The drive to Milford Sound itself passes through unspoiled mountain landscapes before entering the 1.2 km Homer Tunnel which emerges into rain-forest-carpeted canyons that descend to the sound. The winding mountain road, while of high standards, is very prone to avalanches and closures during the winter half of the year.
The long distance to the sound means that tourist operators from Queenstown all depart very early in the day, arriving back only late in the evening. This ensures that most tourists visit Milford Sound within a few hours around midday, leading to some congestion on the roads and at the tourist facilities during the main season. The peak-time demand is also the reason for the large number of tour boats active in the sound at much the same time.
Over the years, various options of shortening the distance to Milford Sound from Queenstown have been mooted, including a gondola route, a new tunnel from Queenstown, or a monorail from near Lake Wakatipu to Te Anau Downs. All would reduce the current round-trip duration (which has to travel via Te Anau), thus allowing tourism to be spread out over more of the day. While a gondola is considered to be out of the running after the DOC declined approval for environmental reasons, the tunnel and the monorail proposals have applied to the Department of Conservation for concessions for land access.
There are a variety of plane charter companies that fly to Milford Sound. Most of the companies fly out of Queenstown International Airport. These companies include: Glenorchy Air, Air Milford, Air Wakatipu, Air Fiordland and Milford Sound Scenic Flights.
On 8 February 2004 a spill of 13,000 litres of diesel fuel was discovered, resulting in a 2-kilometre spill which closed the fiord for two days while intensive cleanup activities were completed. Apparently a hose was used to displace the fuel from the tanks of one of the tour vessels, and various government officials claimed it appeared to be an act of ecoterrorism motivated by rising numbers of tourists to the park, though more details did not become known. The spill was removed and wildlife impact appears minimal.