Introducing Aso-san Area
Halfway between Kumamoto and Beppu lies the Aso-san volcanic caldera. It's the world's largest (128km in circumference), so big that it's hard at first to get a sense of its scale, and strikingly beautiful. Formed through a series of eruptions over the past 300,000 years, the current outer crater is about 90,000 years old and now accommodates towns, villages and train lines.
Aso-san is still active, and the summit is frequently off-limits due to toxic gas emissions or wind conditions. Check with the tourist information centre or www.aso.ne.jp/~volcano/eng/ for updates in English.