Boiling Red Spring at Umi Jigoku
Umi Jigoku is known for it's sea-blue pool, but I found the more natural (and less steamy) rust coloured pool more attractive.
Blue Pool of Hell
Umi Jigoku is known for it's sea-blue pool, but the grounds and surround gates are also worth visiting. The pool was hard to photograph - there is so much steam, the pool and the color is often obscured.