Millford Sound Discovery Centre

Milford Sound has a very particular ecology. The water streaming off the hills is rich in tannin. It is also very cold as it is coming off glaciers. As cold water is less dense than water over 4C this means that there is a non-mixed layer of murky brackish water overlying the seawater. Consequently the light levels mimic those nomally found at much deeper  depths and  such reef creatures as black coral and various fish spies are found at 15m rather than 150m. To exploit this phenomenon for tourism an underwater observatory was built with an artificial reef surrounding it. It opened in the 1990s but they had been growing the reef since 1982, including transplanting 150 year old specimens  of black coral. (Black coral is actually white,the Kiwis are good at names, but apparently goes black when it's dead).