Rotomairewhenua: the clearest body of fresh water known to man

Rotomairewhenua, also known as the Blue Lake of New Zealand’s Nelson Lakes National Park, officially holds the title of the clearest lake in the world.
Literally translated as the “land of peaceful waters”, Blue Lake is spring fed by the neighboring glacial Lake Constance, and its water passes through a natural debris damn formed a long time ago by a landslide.
This debris acts as a natural filter that retains most of the particles suspended in the glacial water, making Blue Lake almost as clear as distilled water.

New Zealand’s National Institute of Water and Atmospheric Research (NIWA) carried out scientific tests of the water and determined it to be the clearest natural body of fresh water known to man.
NIWA hydrologist Rob Merrilees was the first to recognize the optically outstanding characteristics of Blue Lake, comparing it to the country’s famously clear Te Waikoropupu fresh water springs.
It was only after a 2009 visit to Blue Lake that he realized it could be even clearer than Te Waikoropupu, and two years later, NIWA testing confirmed that to be true.

Research has shown that visibility in Blue Lake is between 70 – 80 meters, which makes it almost as optically clear as distilled water.

“The theoretical visibility in distilled water is about 80 meters, as estimated from the best available instrumental measurements in the laboratory,” Dr Davies-Colley said. “So Blue Lake is a close approach to optically pure water”.

Blue Lake lies within the rohe (tribal area) of Ngāti Apa ki te Rā Tō.
As part of the iwi’s 2010 treaty settlement, the lake passed into tribal ownership and was then given back to the Crown. The lake was traditionally used in ceremonies to cleanse the bones and release the spirits of the dead, so they could begin their journey to Hawaiki, and the iwi regard its waters as tapu (sacred).
Interestinhgly, Blue Lake was used only for males, while Lake Constance for females.
Luckily, it is considered sacred by the local Māori tribe, and no one is allowed to enter, let alone bathe in its clear waters, with one notable exception: in early 2013, Danish photo-journalist and environmentalist Klaus Thymann was granted special permission by the Māori, NIWA and New Zealand’s Department of Conservation to enter Blue Lake and capture its astonishing clarity on camera, for conservation purposes.
And, of course, the photos he took are simply breathtaking.