Mer de Glace 2

The Mer de Glace ("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.

That said, it receded pretty bad; I've seen some photos from 1911 or so where it would've been much closer to the viewing platform
There was a guide there who told us that at some point, the ice was 300 meters deep (it's about 30 now) by the platform while still 300m at that back side.
But you can still see some blue ice in little crevices below (row 1 photo 4)

After marveling at this nature phenomenon (or what's left of it), we took the gondola down to Ice CavesBaca lagi