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Blue Lake, a remote sub-alpine lake, in Nelson Lakes National Park, New Zealand, has exceptional optical purity. Horizontal sighting range ('visibility') on six visits approached 80 m, the theoretical limit based on accepted light-attenuation coefficients for pure water, making Blue Lake the most visually clear freshwater yet reported. Blue Lake is characterized by blue-violet hues seen only in the clearest natural waters. Light penetration is as expected for pure water, except in the ultraviolet to blue spectrum where light-attenuation is increased slightly by very low ('oceanic') concentrations of colored dissolved organic matter. Particulate concentrations are extremely low, comparable to those in the Southeast Pacific Gyre, and the lake is hyper (ultra) oligotrophic (Chlorophyll a concentrations ~ 0.014 mg m-3). Continuous monitoring of beam attenuation (532 nm) over 18 months demonstrated consistently high visual clarity, except for short-lived episodes following turbid inflows from ephemeral streams. The dominant inflow to Blue Lake comes from springs emerging from the face of landslide debris separating the lake from nearby glacial Lake Constance. The optical purity of Blue Lake apparently reflects efficient removal of particles and dissolved organic matter during groundwater flow from Lake Constance to these springs. Blue Lake may be valuable in aquatic optics research for testing optical instruments and models. [ABSTRACT FROM AUTHOR]