If you want to get an idea for yourself of the activity of Mt Aso, hop on the cable car that sets off a few miles from the Volcano Museum and takes you to the Naka-dake Crater. The landscape of black basaltic rocks is permanently shrouded in gassy emissions, which mask an immense green lake with acid waters bubbling at a temperature of around 320°C. While you are there, head to the top of Taka-dake, from where there is a superb view of the whole caldera.