Mer de Glace, Chamonix
Concierge.com's insider take:
The massive, desolate glacier known as Mer de Glace is 4.3 miles long and moves 295 feet per year. In summer, a number of gorgeous walks start here; in winter, you can descend to it through the steep Vallée Blanche from the Aiguille de Midi on skis. If you're not up for the steep slopes, take the train from Chamonix to Montenvers and admire the views from there. Afterward, take the cable car from Montenvers to La Grotte de Glace, a fairy-tale cavern hollowed out of the glacier, or learn more about the place's geology at the Crystal Gallery next to the station. The train to Montenvers-Mer de Glace departs frequently from Chamonix (the station is behind the Gare SNCF in the town center).