Japan is known as a volcanic country. Sakurajima island, about 850km away from Mt. Fuji, is said to have a large eruption before long. Around Mt. Fuji area, Mt. Ontake erupted last September, and Hakoneyama has been increasing its activity since June.
When you come to Japan for climbing, check these websites in advance.
Japan Meteorological Agency : http://www.jma.go.jp/en/volcano/
Volcano Discovery : http://www.volcanodiscovery.com/japan.html