A fiord, not technically a sound
Despite its name, Milford Sound is technically a fiord — a valley carved by glaciers and later flooded by the sea, rather than a river-formed sound. It sits within Fiordland National Park, part of the Te Wahipounamu UNESCO World Heritage Area, in the remote southwest corner of the South Island. Mitre Peak, rising sharply from the water to around 1,600 metres, is the fiord's best-known landmark and the subject of most of its classic photographs.
One of the wettest places on Earth — and better for it
Fiordland receives extraordinarily high rainfall, among the highest of any inhabited region in the world, as moist Tasman Sea air is forced upward by the Southern Alps. Rather than a reason to avoid it, rain is often when Milford Sound is at its most dramatic — heavy rainfall triggers dozens of temporary waterfalls cascading directly off the surrounding cliffs, in addition to the fiord's two permanent falls, Lady Bowen and Stirling.
Getting there is part of the experience
Milford Sound has no town of any size and is reached via the Milford Road, a scenic but demanding drive through Fiordland from Te Anau, the nearest sizeable settlement. Most visitors see the fiord itself via a boat cruise, which is the only practical way to take in its scale — the cliffs and waterfalls are far taller than they read from any single viewpoint on land.
A long day from Queenstown
Queenstown is the most common base for a Milford Sound day trip, but it's a genuinely long day — several hours' drive each way on top of the time spent at the fiord itself — so an overnight stay near Te Anau is worth considering for a less rushed visit.