Mer De Glace drains the north side of Mont Blanc. This is the largest glacier in this section of the Alps, it is 12 km long. The “sea of ice” terms not only refers to the size of the glacier, but also to the ogives, curved color bands formed at the base of the icefall. This sea of ice is slowing down as well as thinning and retreating. This has led to the lowest 12% of the glacier being stagnant and appears ready to melt away in the coming decades. A new paper Vincent et al (2014) model Mer de Glace into the future and generate a retreat of 1200 m by 2040, this is likely a minimum. 
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Mer de Glace
From a Glaciers Perspective 