The Five Mountains of Aso are the smaller mountains within the outer rim: Eboshi-dake (1337m), Kijima-dake (1321m), Naka-dake (1506m), Neko-dake (1408m), furthest east, and the highest, Taka-dake (1592m).
Naka-dake is the active volcano: very active in recent years, with fatal eruptions occurring in 1958 and 1979, and other significant eruptions in 1989, 1990 and 1993.
If Naka-dake is behaving, a cable car whisks you up to the crater's edge in just four minutes; from there, it's a 30-minute walk.
It'll cost ¥600 in tolls and parking if driving yourself. The cable car is 3km from the Aso Volcano Museum. Don't miss the 100m-deep crater – with pale green waters bubbling and steaming below, it varies in width from 400m to 1100m, and there's a walk around the southern edge of the crater rim. Arrive early in the morning to glimpse a sea of clouds hovering inside the crater, with Kujū-san (1787m) on the horizon.