Blue Lake (Rotomairewhenua), which is part of Nelson Lakes National Park, boasts the cleanest freshwater available anywhere. As optically clear as distilled water, scientific examinations have revealed visibility down to 80 meters deep. Underground springs replenishing the lake with water filtered through layers of rock over decades produce this amazing clarity. Like the sky, the lake's blue look results from its scattering and absorption of varying light wavelengths. The lake's remarkable purity has been preserved in great part by the surrounding natural beech forest and alpine surroundings, which have been mostly un altered. The lake's holy significance among the local Māori people has helped to preserve it; ancient customs limiting particular activities close to the lake help to reflect this. Water flowing through several rock layers, each functioning as a natural filter eliminating pollutants and particles, feeds Blue Lake using a unique geological filtering mechanism. The lake's water clarity surpasses that of laboratory-grade distilled water, according to research, so it is a useful subject for scientific investigation. The ecosystem of the lake is quite sensitive; specialist bacteria fit the ultra-pure conditions help to preserve its remarkable clarity.
