Japan/Hokkaido: Mt Loh aka Sulphur Mointain Fumaroles
There is a whole mini-mountain in the Akan National Park that is basically made out of sulphur – there are about 50 fumaroles billowing white hot steam into the air coming from acid-fluorescent colored yellow vents. The air is putrid with strong sulphur, but you can still walk up pretty close to these gates of hell.