With global warming, the glaciers are melting. Once stretching to the edge of town, they now end high in the mountains. Moreover, their greenish glacial water is forming lakes. In summer, when the melting accelerates, floodwaters threaten the area. But the avalanche witnessed by Mr. Bomio shows that the shrinking of the glaciers removes a kind of buttress supporting parts of the mountains, menacing the region with rock slides.

Grindelwald stands as a stark example of what is happening these days to Switzerland’s glaciers, and there are more than a hundred, large and small. As the Lower Grindelwald Glacier shrank, its ice no longer buttressed the east wall of the Eiger, a 13,025-foot mountain that is part of the ring south of Grindelwald. Moreover, the warming reduces the effect of permafrost that once acted as a sort of glue binding together the mass of the mountains. On that day in 2006, a chunk of the Eiger amounting to about 900,000 cubic yards fell from the east face, causing the cloud of rock dust that startled Mr. Bomio and his friends.

Since 1997, Ruth Meier has run the Hotel Gletscherschlucht (the name means glacier gorge), with 6 rooms and 18 beds, at a point where water from the melting lower glacier runs out of a steep and narrow gorge. Well into the 20th century, the glacier extended clear through the gorge, which is about three-fifths of a mile long, and until about World War I, ice blocks were carved out of it for use in cooling in restaurants and kitchens as far afield as Paris. Where her hotel stands a field kitchen once fed the workmen who hacked the ice.

But now a large lake of melted glacial water has formed above the gorge. To avoid potential flooding that would threaten the village below, Ms. Meier said, a $15 million tunnel, more than a mile long, was completed in 2010 to channel excess water when the lake swells in the summer. Before that was done, she said, summer floodwaters regularly pushed gigantic ice blocks down the gorge.

“In July and August, it sounded like battle tanks coming down,” she said, sipping mineral water. “You could hear the stones rolling.” Floodwaters forced their way through the narrowest parts of the gorge, about 30 feet across, “like water gushing from a garden hose,” she said.