On the majestic Marmolada, the Queen of Italy’s Dolomites, the first Sunday of July was a beautiful day for hikers—the sky almost cloudless, a warm 82 degrees Fahrenheit in the valley. But for the mountain, even the 50°F near the 11,000-foot peak was sweltering. From its glacier, the largest in the mountain range, a section the size of two football fields broke off. Ice and debris thundered down with the force of a collapsing skyscraper. Eleven people—two of them experienced mountain guides—never made it home.

“I saw pictures of what it looked like before the collapse, and I would have taken my own son there that day,” says Alberto Silvestri, an Italian guide. For alpinists and locals, the tragedy was a