France’s largest glacier, the "Sea of Ice," flows 7km down the northern side of Mont Blanc, scarred with crevasses formed by the immense pressure of its 90m-per-year movement. The Train du Montenvers, a picturesque, 5km-long cog railway opened in 1909, links Gare du Montenvers with Montenvers (1913m), from where a cable car (free) and then 580 stairs descend to reach the glacier and the Grotte de Glace.
On a more somber note, look out for plaques on the walk down indicating the height of the glacier between 1985 and 2018 – it's one thing to read about climate change and shrinking glaciers, but seeing it in person is particularly alarming. Because every year more steps need to be added to reach the glacier (there were only three in 1988), glacier access will likely be relocated 500m up valley in the near future.
On foot, the Mer de Glace can be reached from Plan de l’Aiguille on the Grand Balcon Nord trail. The two-hour uphill trail from Chamonix starts near the summer luge track. Traversing the crevassed glacier requires proper equipment and an experienced guide.
