Mer de Glace, Valley glacier in Chamonix-Mont-Blanc, France.

Mer de Glace is a valley glacier in the Mont Blanc massif above Chamonix, extending roughly 7 km in length with a thickness of around 200 m. The ice moves slowly downhill and forms crevasses and serac towers that shift constantly.

In 1741 William Windham and Richard Pococke reached the glacier from Chamonix and named it Mer de Glace. Horace Bénédict de Saussure later observed it and had Marc-Théodore Bourrit promote it, drawing writers, aristocrats and scientists. Three refuges were built at Montenvers, and the Montenvers railway from Chamonix opened at the start of the 20th century.

The name translates to Sea of Ice and dates back to the 18th century, when travelers compared the wavy surfaces to a frozen ocean. The Grotte de Glace is carved afresh each year and features sculptures and furniture shaped from clear blue ice.

The glacier is reached from Chamonix by the Montenvers rack railway, which climbs to the Montenvers station overlooking the ice. From there, a gondola and then a long metal staircase lead down toward the ice cave; the staircase grows longer as the glacier retreats. The cave is carved anew each year, and the underground hydroelectric plant has used meltwater since 1973.

Markers along the rock walls show former ice levels and illustrate the retreat of the ice over decades. The flow rate reaches around 90 m per year as the mass moves downhill.