Naka-dake (1506m) is Aso's active volcano – very active in recent years, with fatal eruptions in 1958 and 1979, and other significant eruptions in 1989, 1990, 1993 and 2016. Access to the crater area may be closed when there's an increase in volcanic activity, such as dangerous sulphurous-gas emissions. Check the website for current conditions. People with asthma, bronchitis or head colds are advised to stay away.
If you make it to the top, you'll be rewarded by stark rock faces and a fluorescent-green lake inside the 100m-deep crater. With waters bubbling and steaming below, the crater varies in width from 400m to 1100m, and there's a walk around the southern edge of the rim. Arrive early in the morning to glimpse a sea of clouds hovering inside the crater, with Kujū-san (1787m) on the horizon.
A cable car served the crater until the most recent eruption, but operations have since been suspended. Nowadays, it's ¥800 for parking; alternatively, you can park at the former cable-car station and hike up in about 30 minutes, or take a shuttle bus (¥1200 return).
The most expensive, and perhaps most thrilling, way to see the crater is from way above, in a plane operated by Saga Aviation.