Milford Sound is in fact a Fiord created by glacial action which has shaped the deep u shaped valleys and steep cliffs. It was inaccurately named by the early explorers as a Sound. With up to 700 cm's of rainfall annually it is one of the wettest places on earth and a very unique environment. The rainfall becomes stained with tannins and minerals flushed from the rocks and lush forests above. The fresh water sits on top of the seawater in the Fiords allowing deep-water species to live much closer to the surface. This fresh water layer, up to 5 metres deep, filters out the light to create unusually dark conditions in the sea water below. The underwater observatory at Harrison's Cove allows you to go 8 metres below the surface to view a cross section of species that live happily in Milford Sound. Enjoy Milford Sound .