Halfway between Kumamoto and Beppu lies the volcanic caldera of Aso-san (阿蘇山). It's the world's third largest (128km in circumference) and strikingly beautiful, yet its scale is so big that it's hard at first to comprehend. 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 lush green farmland. Aso is the main town, and nearby Uchinomaki has inns, shops and dining. To the southeast, the town of Takamori is more intimate and charming.
The Aso-san range is still active, and the summit is frequently off-limits due to toxic gas emissions or strong winds. 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 before planning a trip.