Mount Ontake in Japan rises 3,067 meters above sea level — a windswept giant standing head and shoulders above densely forested hills. This ancient volcano is a popular trekking site. A trail traverses its ash- and boulder-strewn ridges. There are several huts and a shrine. On September 27, 2014, hikers took advantage of a blue sky and gentle wind. At 11:52 a.m., over a hundred of them stood on the summit, eating snacks and taking photos. Disaster struck with little warning.

The windows and doors of a nearby hut rattled, vibrated by a low-frequency shock wave inaudible to humans.

People glanced around curiously and quickly saw it — half a kilometer down the southwest slope, a gray cloud billowed from the mountain.

The ash cloud swept over the summit with a blast of hot air, leaving people shaken and blinded, but otherwise unhurt. Disoriented in that gray fog, they couldn’t see what arrived soon after.

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