Mount Aso, Kyushu

Mount Aso is a huge caldera with the peak of Aso in the middle. That explains where you are: in the centre of a huge active volcano - although the original magma chamber is empty.
You can climb on top of the peak if the concentration of sulphur in the air is not too high. You should not underestimate it. There are also some concrete shelters for obvious reasons but they won't protect you from the sulphur vapour.

Apart from that, you can visit the museum not far away from the crater. There you can operate remote-controlled cameras that permit you a view into the crater. Furthermore, you can also look from the mountain into the caldera with its endless rice fields or catch a ride on a train through these fields.