TOKYO — Huge boulders falling from the sky. Billowing gray smoke that cast total darkness over the mountain. Volcanic ash piling on the ground and fumes filling the air.

Some survivors of the eruption of Mount Ontake made a split-second decision to hide behind big rocks or escaped into lodges that dot the mountain’s slopes. Outdoors, other hikers fell, hit by rocks or possibly suffocated by gases, and quickly buried in ash. At least 36 people were killed in Saturday’s surprise eruption.

Some bodies have been recovered, and search efforts for the other missing bodies resumed Wednesday morning.

For survivors such as mountain guide Sayuri Ogawa, it was a near-death experience. The experience she recalled on Tuesday and the accounts of others suggest that luck and instinct made the difference between life and death for the hikers who were in harm’s way.

Despite its impressive plume, the eruption was not a major one with lava flow. Yet, it proved deadly, because so many people were at the summit on a perfect day to enjoy hiking and the autumn leaves.

The eruption caught hikers by surprise. Seismologists had detected signs of increased seismic activity at Mount Ontake, one of Japan’s 110 active volcanos, but nothing signaled a fatal eruption.