Aletsch Glacier, Valley glacier in Jungfrau-Aletsch protected area, Switzerland.

The glacier stretches 23 kilometers through the Bernese Alps, forming the longest ice stream in the entire Alpine region. Its surface shows a pattern of crevasses, exposed rock debris, and deep ice channels that shift in appearance under changing light throughout the day.

During the last ice age around 18,000 years ago, ice covered the ridges between Bettmerhorn and Riederhorn completely, leaving only the highest peaks visible above the frozen surface. Naturalists in the 19th century began mapping and measuring glacial movement, documenting the slow changes that continue today under warmer conditions.

The name Aletsch comes from the alp at about 1,845 m, first recorded as Alech in 1231; the present form appears in 1460, and its origin is unknown. In Valais the word is stressed on the first syllable, Áletsch, unlike the rest of German-speaking Switzerland. The glacier feeds the Massa, which drove a hydroelectric plant at Bitsch above Naters.

Visitors reach viewing points at Eggishorn, Bettmerhorn, and Moosfluh through cable cars that run during the main season. Early morning or late afternoon offers the clearest views, as fog is less common during those hours and the changing light brings out details in the ice surface below.

At Konkordiaplatz, where four smaller glaciers merge, ice beneath the surface reaches a thickness of roughly one kilometer. Under that weight, the ice creeps downhill at a rate of several dozen meters each year, carving channels and forming moulins as it moves.