Someone has to be lucky from time to time, I suppose, and on Wednesday we were the lucky ones.

The Perito Moreno Glacier in the southern Andes flows for 30 km from the Southern Patagonian Ice Field into Lago Argentino, where it isolates a section of the lake. Every four years or so, as the water rises by as much as 30 metres behind this natural dam, and as the ice melts and weakens in the summer months, the wall fails and a spectacular rupture occurs. It starts as a narrow tunnel from the higher section to the lower section of the lake, but as the water flow increases, the tunnel widens, and ice tumbles ever more rapidly into the flow, until finally the whole arch fails.

Watch the final spectacular collapse here.

The rupture began on Tuesday, and we saw the water in full flow on Wednesday on a bright, windless afternoon, after a highly supervised 90-minute trek, with crampons, on the surface of the glacier (more dangerous, arduous and difficult than you imagine). The final failure happened yesterday, watched by around 4,000 tourists. We weren’t there. We were lucky, but not that lucky.

When the winter snows fall and the temperature drops, the ice will advance and dam the lake once more, and the four-year cycle will begin again.