The Mer de Glace — Sea of Ice — is the largest glacier in France and the second largest in the Alps. It is 12 kilometres long, up to 400 metres deep, and contains an estimated four billion cubic metres of ice. The easiest way to reach it is by the Montenvers rack railway, which climbs from Chamonix station to the Montenvers viewpoint at 1,913 metres in around 20 minutes.
From the Montenvers station, a gondola descends to the glacier surface, where an ice cave is carved each year into the body of the glacier itself. The cave is refashioned annually because the ice moves roughly 90 metres per year. A walkway through the blue-green interior passes sculpted chambers and frozen waterfalls. Back on the surface, the scale of the glacier — and of its retreat — is unmistakable: metal stakes driven into the ice in previous years are now metres above the current surface, measuring a loss that has accelerated sharply since the 1990s.