The Five Mountains of Aso are the smaller mountains within the Aso-san caldera's outer rim: Eboshi-dake (1337m); Kijima-dake (1321m); Naka-dake (1506m); Neko-dake (1408m), furthest east; and the highest, Taka-dake (1592m). Access roads were damaged in the 2016 earthquakes and at the time of writing were in the process of being repaired, although some areas may be closed due to toxic gas emissions.
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, 1993 and 2016.
It will cost ¥800 in tolls and parking if you're self-driving. The most expensive, and perhaps most thrilling, way to see the crater is from way above, in a plane operated by Saga Aviation.