OTAKI, Japan — Gaku Harada remembers it as a perfect day for hiking. A clear blue sky drew hundreds of weekend climbers to Mount Ontake, one of Japan’s most celebrated peaks, to see the first tints of autumn in the leaves.

Then, without warning, the top of the mountain exploded.

Mr. Harada, a professional climbing guide, was leading a local television crew up the mountain to film a nature show. One moment, the peak was clearly visible about a mile in front of them. The next, it vanished into a dark, billowing cloud as a thundering wall of gray ash raced down the slope toward them.

Unable to process what he was seeing, he said he froze, then snapped out of it when a companion yelled, “Eruption!” Within minutes, his group was engulfed in ash so thick it blotted out the sun and began to fill their mouths. They groped their way down the mountain in the unnatural darkness as the sickening, rotten-egg stench of sulfur filled the air. But it was the sounds, he said, that scared him most: the thunder from the eruption, and the thud of boulders crashing into the slope behind them.

“I thought it was the end of the world,” said Mr. Harada, 38, who helped lead the group to a lodge on the mountainside. “I had only seen volcanic eruptions in movies and never dreamed I’d experience one in real life.”