One of the first, this two-car cogwheel train — inaugurated in 1909 — transported turn-of-the-century visitors to the edge of the Mer de Glace glacier. The Mer de Glace is France's largest glacier — four miles long. In the 1600s, the glacier extended much farther downhill — actually threatening to block off the valley. For me, this shrinking glacier is one of the most poignant. When I first came here, back in the '80s, the Mer de Glace was hundreds of feet higher than it is today.