Blue ring, red cone.

Sindri is a small scoria cone perched atop Tindfjallajökull in the southern Highlands of Iceland. For a brief period in June, the crater lake acquires an extraordinary blue ring as snow and ice begin their retreat.

Timing is everything here. Arrive too early and the cone remains largely buried beneath snow. Arrive too late and much of the snow has vanished or lost its freshness.

On this occasion, the elements aligned. The striking colour of the water arises from the selective absorption of light by water and ice, which preferentially remove the longer red wavelengths, allowing the shorter blue ones to dominate the light scattered back to the observer.

The accompanying video follows the departure from Reykjavík and the flight into the Highlands, culminating in an orbit around the crater.