The Mer de Glace (French pronunciation: [mɛʁ də ɡlas], lit. 'Sea of Ice') is a valley glacier located on the northern slopes of the Mont Blanc massif, in the French Alps . It is 7.5 km long and 200 metres (660 ft) deep but, when all its tributary glaciers are taken into account, it can be regarded as the longest and largest glacier in France, and the second longest in the Alps after the Aletsch Glacier .

I can no otherwise convey to you an image of this body of ice, broken into irregular ridges and deep chasms than by comparing it to waves instantaneously frozen in the midst of a violent storm.

The glacier lies above the Chamonix valley. The pressure within the ice is known to reach at least 30 atmospheres. The Mer de Glace can be considered as originating at an elevation of 2,100 metres (6,900 ft), just north of the Aiguille du Tacul , where it is formed by the confluence of the Glacier de Leschaux and the Glacier du Tacul . The former is fed by the Glacier de Talèfre , whilst the latter is, in turn, fed by the Glacier des Périardes, the vast Glacier du Géant and the broad icefields of the Vallée Blanche.