A popular hiking site in central Japan turned into a death trap Saturday when a volcano erupted. At least 31 people are believed to have been killed at Mount Ontake, though authorities have only begun recovering the bodies. Here's a summary of events and a guide to understanding them:



Mount Ontake erupted at perhaps the worst possible time. It was a beautiful fall Saturday with clear blue skies, shortly before noon — a perfect time for a hike. At least 250 people are believed to have been on the upper reaches of the 3,067-meter (10,062-foot) peak when it erupted with thunder-like booms, sending a steady plume of gas and ash skyward and over the surrounding area. It's unclear how the victims died, but survivors said the plume was so large that it blocked out the sun, plunging them into total darkness for a time.

It's not necessarily the size of the eruption that matters, but also the volcano's proximity to people, Nagoya University volcanologist Koshun Yamaoka said at a news conference Sunday. "Even small eruptions can cause major damage if people are around," he said. "And the problem is that catching signs of such small eruptions is difficult."



Although somewhat increased seismic activity had been recorded at Mount Ontake for about two weeks, there were no indications of a major eruption, said Satoshi Deguchi, a Japan Meteorological Agency official in Nagano prefecture. Typical signs, such as increased seismic rattling or underground structural movement, were not detected.



With 110 active volcanoes, Japan has eruptions from time to time, but fatal ones have become relatively rare. The last one was in 1991, when 43 people were killed by an eruption of Mount Unzendake in Kyushu, Japan's southernmost main island.

There have been about a dozen fatal eruptions in Japan since 1900, but only two since 1960. Besides the 1991 blast at Mount Unzendake, three people died when Mount Aso erupted in 1979, also in Kyushu. Earlier major ones include:

— 1952: 31 deaths at Bayonnaise Rocks, a Pacific island south of Tokyo.

— 1926: 144 killed at Mount Tokachidake on Hokkaido, Japan's northernmost main island.

— 1914: 59 killed at Mount Sakurajima in Kyushu.

— 1902: All 125 residents killed on Izutorishima, a volcanic island south of Tokyo.

— 1900: 72 killed at Mount Adatara in Fukushima prefecture.



If the deaths are confirmed, this would be the first fatal eruption at Mount Ontake in modern times. A similar eruption occurred in 1979, but no one died. However, 29 people were killed in 1984 when a magnitude-6.8 earthquake triggered major landslides in the area, including one at Mount Ontake.