The Aletsch is the largest body of ice in the Alps, a frozen river twenty kilometres long and as much as 900 metres thick, winding down through the Swiss canton of Valais. You meet it from above. A cable car lifts you from Fiescheralp to the Eggishorn at 2,869 metres, the highest viewpoint here, and the whole curve of the glacier unwinds below. On a clear day the Matterhorn and the Bernese giants stand round the skyline behind it. A marked balcony path runs along the crest above the ice, linking the Bettmerhorn with Hohfluh by way of Moosfluh, with the glacier beside you the whole way. Moosfluh is the one place that looks straight up the ice to Konkordiaplatz, where its four arms merge, and that is where the length registers all at once.
Getting onto the ice takes crampons and a mountain guide, and roped up you pick a way between blue crevasses and the rubble-streaked moraines of the upper glacier. It is the only way to feel the scale from the surface rather than a ridge. None of it is standing still. The ice creeps downhill by as much as 180 metres a year, and the dark stripes running its length are the rubble it carries. That rock fell onto the snowfields high up and was funnelled into lines wherever two arms joined; what lies near the tongue came off the mountains over a century ago. At the top end a cog railway tunnels through the Eiger and the Mönch to the Jungfraujoch, Europe's highest railway station at 3,454 metres, with terraces looking down on the snowfields the glacier comes out of. The glacier is also going backwards. It has lost more than three kilometres of length since 1850, and pale bare rock lines the valley walls where the surface used to be. The forest beside it is made of wind-bent Swiss stone pines, a few of them perhaps a thousand years old, and its youngest trees are taking root on raw rock the ice gave up not long ago.