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Gunma is home to a world famous volcano: one of the great sites of the Pacific Rim of Fire that includes celebrated volcanos in the Phillipines and Hawaii.

[Eruption of Asamasan in 1973 - photo by E. Koyama, Asama Volcano Observatory]

Mount Asama is the largest and most volatile of Japan's 108 active volcanos. Asamasan's last major eruption was on Monday, 15 November 2004 at 20:59, only six weeks after its violent eruption on 1 September 2004. A now famous photograph made by the American NASA Earth Observatory satellite shows the white, steamy plume blowing south over Honshu.

Asamayama has a complex volcanic structure. It consists of a young stratovolcano with two craters lying on top of a shield volcano, which then lies on top of an older stratovolcano. A stratovolcano forms into a steep, classic cone shape from viscous lava flows and other magma composites. Stratovolcanos emerge for successive eruptions over hundreds of thousands of years. Fujiyama, Etna, and St. Helens are all stratovolcanos. A shield volcano has comparatively gentle slopes and an over all convex shape. Shield volcanos are known for their formation of large land masses as thin, basalt lava flows from fissures in their summits. Moana Loa and Kilauea on the Hawaiian Islands are shield volcanos.

Climbing Asamayama's caldera (the dual hot craters you want to see) are currently closed because of poisonous fumes leaking from multiple fissures. Should that change, a hiking path is described in an article by Randy Biddle, which can be found here: http://www.outdoorjapan.com/columns/backroads-2a.html

Sightseeing. The more beautiful side of Asama faces Gunma, of course. But frankly, to spend the day doing Asama things, you probably want to go to Nagano side and explore the Asama Volcano Museum (see link below).