Umi Jigoku (Sea Hell)

Umi Jigoku, the "Sea Hell," is the largest and most celebrated of Beppu's seven jigoku, the geothermal hot springs that together form a nationally designated Place of Scenic Beauty. Its name comes from the astonishing cobalt-blue colour of the pond, a hue produced by dissolved iron sulphate rather than any pigment. The water sits at roughly 98 degrees Celsius and belches thick columns of white steam that drift across the surrounding gardens, so the hells here are for viewing rather than bathing - a rule that applies to all seven.

The spring was created some 1,200 years ago by the volcanic eruptions of nearby Mount Tsurumi, and today it anchors a surprisingly refined tourist complex. Boardwalks lead visitors past the main blue pond to a smaller blood-red pond, a lily-filled greenhouse where giant Amazonian water platters (Victoria amazonica) grow in the naturally heated pools, and a vermilion torii gate and small shrine that make for one of Beppu's signature photographs. Steam from the spring warms the greenhouses year-round, letting tropical plants thrive in the cool Kyushu highlands.

Umi Jigoku is the official starting point of the Beppu Jigoku Meguri, the self-guided "hell tour" that strings all seven together. Five of the hells, including this one, cluster within a few minutes' walk of each other in the Kannawa district; the remaining two lie a short bus ride north in Shibaseki. Because it is the first stop, Umi Jigoku tends to be busiest between 10am and noon, so an early arrival rewards you with quieter boardwalks and cleaner reflections on the blue water.

Beyond the ponds, the site leans into its geothermal theme. A footbath (ashiyu) lets you rest tired feet in genuinely hot spring water, and food stalls sell onsen tamago - eggs slow-cooked in the mineral steam - along with steamed puddings and locally made jigoku-mushi snacks. The gift shop stocks yunohana bath salts harvested from Beppu's springs, a practical souvenir that lets you carry a little of the hell home.

The experience is as much about the atmosphere as the individual sights: the constant hiss of escaping steam, the sulphur tang in the air, and the sense that the ground itself is alive. Photographers should aim for a bright, still morning when the blue is most vivid and the steam rises in clean vertical plumes. Autumn adds colour from the maples around the greenhouse ponds.

Getting there is easy from Beppu Station by frequent Kamenoi buses, and the flat, well-paved paths make Umi Jigoku one of the more accessible hells for visitors with limited mobility, though a few gentle slopes and steps remain. Allow around 40 minutes here, or longer if you linger over the footbath and eggs. It is the perfect introduction to why Beppu, sitting atop one of the most prolific geothermal fields on Earth, has been Japan's onsen capital for centuries.