Halfway between Kumamoto and Beppu lies the Aso-san (阿蘇山) volcanic caldera. It's among the world's largest (128km in circumference), so big that it's hard at first to understand 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 is the main town, but Takamori, to the south, is more intimate and charming.
Aso-san is still active, and the summit is frequently off-limits due to toxic gas emissions or wind conditions. In October 2016 the volcano had a major eruption, resulting in severe damage to surrounding areas. Be sure to check the latest access information with the tourist centre or at www.aso.ne.jp/~volcano/eng for updates before planning a trip.