The clearest lake in the world – in pictures

The world’s clearest lake is the Blue Lake – located in the top part of New Zealand’s South Island Photograph: Klaus Thymann /Project Pressure

Scientifically verified reports show visibility of up to 76 metres – compared with distilled water visibility of 70-80 metres Photograph: Klaus Thymann /Project Pressure

The lake is in a restricted conservation area, and entering the lake, let alone diving, is prohibited. It is a 2-3km hike to get to the lake Photograph: Klaus Thymann /Project Pressure

Working closely with New Zealand's Department of Conservation, photographer Klaus Thymann was allowed to conduct the first ever dive in the lake Photograph: Klaus Thymann /Project Pressure

The lake, which in Maori is called Rotomairewhenua, is sacred, and the local Maori tribe, Ngati Apa, was consulted about the dive. Equipment was carefully prepared and a special permit to land a helicopter granted, and on a clear sunny day, the dive was conducted Photograph: Klaus Thymann /Project Pressure

Blue Lake is located 1,200m above sea level and receives water from another lake above it Photograph: Klaus Thymann /Project Pressure

The top lake is above the tree line, meaning its water is relatively unblemished by run-off from overhanging foliage Photograph: Klaus Thymann /Project Pressure

From the top lake, water is filtered through a moraine (glacial debris), before finally creating the Blue Lake Photograph: Klaus Thymann /Project Pressure

This is a hole where water flows out of the Blue Lake. The entire lake is replaced around every 24 hours Photograph: Klaus Thymann /Project Pressure

On rare occasions Blue Lake can become temporarily murky, such as after heavy rain that washes particles into it, but after a few days its astonishing clarity is restored Photograph: Klaus Thymann /Project Pressure