Mer de Glace ice cave, Ice cave in Chamonix-Mont-Blanc, France

The Mer de Glace ice cave is an artificial gallery dug each year into the glacier below the Montenvers viewpoint. A metal staircase of 580 steps zigzags up to the entrance, where white tarps cover sections of the grooved blue-grey ice above. Inside, branched passages lead to rooms with ice sculptures and furniture.

Before the cave existed, visitors came to see the 20–30 metre arch the Arveyron cut at the glacier's edge, which sometimes failed to form. In 1862 a foreign builder proposed a gallery modelled on Grindelwald; Chamonix residents dug their own 25-metre gallery and rotunda instead, opened from April to June 1863 as the "Crystal Cave". From 1872 the cave was dug in the Bossons Glacier as the Mont-Blanc cave.

The cave's name has shifted with its site: the 1863 gallery was billed as the "Crystal Cave" or "Crystal Palace", and from 1872 a cave dug in the Bossons Glacier was called the Mont Blanc cave. Today's galleries hold furniture and ice sculptures, and a chalet at the entrance sold souvenirs from the first season.

Access starts with the Montenvers rack railway from Chamonix, then a gondola lift east of the station, followed by a staircase that reached 580 steps in the early 2020s as the glacier thinned. The cave lies about 200 metres north of the 1988 site; a new lift in 2024 moved it back toward the first Montenvers location. Expect cold, slippery ice underfoot.

The cave is dug anew each year because the glacier flows about 70 metres annually; the layout stabilised from 1953, with branched galleries leading to rooms holding furniture and ice sculptures. In 2024 a new lift on much the same line as the old cable car moved the cave back to the original Montenvers site.