Mount Aso and Sensuikyo-gorge

"Aso-san to Sensui-kyo"The Biggest Active Volcano in the World, Which Is a Barometer of Earth's Power

Outline Aso is the active volcano that consists of huge caldera topography and the vast outer rim of a crater with 18km (11 miles) to east and west, and 25km (16 miles) to north and south, that covers the caldera.
In "Nakadake" (Mount Nakadake), there is a crater with diameter; 600 meters (1,969ft), and milky-green-colored "Caldera-ko" (Caldera Lake) emerges, when the crater is calming down.
It's a sign to start erupting when the Caldera-ko disappears and the crater gets hot.
When you go to the crater of Nakadake in Aso, you can use Asozan-ropeway or Asozan Park Road. If they are not controlling the entry because of the eruption activity, you can look down a dynamic view of the crater which emits a great deal of smoke.

Also, as all these areas are designated as "Aso Kuju National Park", you can enjoy the natural treasure,"Kusasenrigahama" at the foot of the mountain, "Sensui-kyo" (Sensui-gorge) in the Miyama Kirishima Gunrakuchi, and many more onsen places too.