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a system of karst caves in Switzerland, in the canton of Schwyz; the most extensive cave system known in Europe. The total length of the passages in the Hölloch Caves is 123.5 km; the caves are a complex multilevel system that reaches a depth of 740 m. The caves formed in Cretaceous limestones of the northern spurs of the Glarus Alps, in the Muota River valley; they abound in ellipsoid chambers. The largest group of stalactites and stalagmites is concentrated in the Pagoda passageway. The Hölloch Caves are a tourist attraction.