Optically pure waters in Waikoropupu (‘Pupu’) Springs, Nelson, New Zealand

We have made in situ observations of the optical properties of Waikoropupu (“Pupu”) Springs, Nelson, New Zealand. Visual clarity was measured in the horizontal direction over a sight path “folded” using a plane mirror so as to accommodate the sighting range within the 35 m wide main springs basin. An average black‐body visibility of 63 m was obtained, the highest yet reported for any fresh water, and close to the theoretical maximum for optically pure water. Measurements with a beam transmissometer were consistent with the visibility observations. As well as ranking among the very clearest waters in the world, these spring waters are virtually indistinguishable from pure water as regards their spectral irradiance attenuation in the ultraviolet and beam attenuation in the green region of the spectrum, undetectably low content of yellow‐coloured organic material, and remarkable blue‐violet colour.