The Marble Caves of Patagonia — General Carrera Lake, Chile

Halfway along the shoreline of General Carrera Lake, in the Aysén region of Chilean Patagonia, the water has spent the last six thousand two hundred years or so eating its way into solid marble. What is left is a cluster of caverns, tunnels and free-standing columns worn out of the monoliths at the water's edge — known collectively as the Capillas de Mármol, the Marble Chapels, with the largest individual formations carried on the charts as the Marble Chapel and the Marble Cathedral. Nothing here was carved or quarried. Wave action did all of it, working along the grain of the stone, which is why the hollows are smooth and rounded rather than blocky, and why the ceilings curve the way a wave does.

The colour is the part that surprises people, because the marble itself is not blue. General Carrera is a glacial lake, fed by meltwater carrying rock flour so fine it stays suspended, and that suspension is what turns the water the deep turquoise you see at the bottom of the frame. Inside the caverns, light bounces up off that water and onto the polished stone, so the marble takes on the lake's colour and the banding in the rock — laid down long before any of the caves existed — reads as ribbons of blue and white. Change the light or the lake level and the whole interior changes with it. The lake around it is not small: shared with Argentina, where it is called Lago Buenos Aires, it covers about 1,850 square kilometres in total, making it the largest lake in Chile, and its western basin drops to 586 metres deep. Its outflow runs west through the Baker River to the Pacific. The Aonikenk name for it was Chelenko, meaning stormy waters, which tells you the calm surface in this photograph is not the usual condition.

As a jigsaw this is an unusually pleasant build, because the image splits into two halves that behave completely differently. The bottom third is water: rippled, pale, reflective, and sorted almost entirely by the direction the ripples run rather than by colour. The top two-thirds is marble, and that is where the real structure lives — follow the dark banding lines, because they sweep in long continuous curves across the ceiling and act like guide rails through an otherwise repetitive field of blue. Start with the three deep cave mouths, which are the only genuinely black areas in the frame and lock down the middle of the picture fast. The hardest pieces are the mid-tone blues where reflected light fades into shadow along the upper left; leave those until the banding has given you edges to attach them to.