Mer de Glace Deep Dive
The Mer de Glace is the most accessible big glacier in the world. The Montenvers railway from Chamonix has carried visitors to its moraine since 1908. The catch — beautifully visible to anyone who descends to the ice cave today — is that the glacier has dropped over 200 metres vertically since the railway opened.
Geography
The Mer de Glace ("sea of ice") flows from the confluence of the Glacier du Geant and the Glacier de Leschaux down to the Bois valley above Chamonix. Total length about 7 km, surface area roughly 30 km2. It is the second-longest glacier in the Alps after the Aletsch, though far shorter than it once was.
Retreat
Between 1995 and 2024 the snout retreated approximately 1.2 km and the surface dropped over 200 metres at the Montenvers level. The walk from the railway terminus down to the ice cave now involves a long stairway descent — markers on the stairs label the year the surface sat at each level.
How to visit
- Montenvers railway. Cog railway from central Chamonix to the Montenvers terminal at 1 913 m. Year-round operation; check schedule.
- Glaciorium. Permanent exhibition at Montenvers on the glaciology and history of the site.
- Ice grotto. Carved fresh each year. Free with the railway ticket; reached via the new gondola opened in 2024 (replacing the earlier descending escalator system).
- Refuge du Couvercle. Multi-day base for traverses; reached on foot from Montenvers in 4-5 hours.
When to go
Mid-June to mid-September for the ice grotto. The railway runs in winter too, but the grotto is closed; the moraine viewpoint is spectacular under snow.
Why it matters
The Mer de Glace is the world's best-documented glacier — paintings, postcards, photographs and now satellite measurements stretch back over two centuries. Visit with a copy of a 19th-century engraving for the most stark before-and-after experience you can find on Alps ice.
Explore on the map
Find the Mer de Glace, the Montenvers railway and the Couvercle hut on the interactive map. The historic ice cave positions are also marked — you can compare each year's location.