New Zealand was covered in about 2km of ice for 70,000 years, only melting recently (10,000 years ago).  The legacy is these scooped valleys and deep fjords.

Milford Sound is 400 metres deep, with sheer sides and no anchorages.  There is 6m of rain a year, resulting in a 3m layer of fresh, tannin-rich water on the surface of the sea.  This cuts down the light to the extent that species which only survive at 70m in the ocean, can live in the fjord.