The most visually striking of Beppu's nine jigoku (hells), Umi Jigoku is a brilliantly blue pool of boiling water that looks improbably like a tropical sea. The colour comes from dissolved minerals and the steam that perpetually drifts across the surface adds to the otherworldly effect. The surrounding garden is well-maintained, with a large koi pond fed by cooled spring water and lotus hothouses warmed by the steam.
Umi Jigoku is in Kannawa district, where most of the other jigoku are also concentrated. A combined day pass covers eight of the nine hells and is the economical choice if you plan to visit more than two or three. The spring itself is enclosed by low fencing — this is not a bathing hot spring — but you can cook eggs and corn in baskets lowered into the steam at vendor stalls nearby.