France’s largest glacier, the 200m-deep '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 descends to the glacier and, 420 stairs later, the Grotte de Glace. Also worth a visit is the Glaciorium, an exhibition on the formation (and future) of glaciers.
Along the walk down to the cave, there's a bird's-eye view of daredevil skiers disappearing into the icy void. It's eerie to note the signs along the walk, marking the gradual retreat of the glacier over decades.
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.