Oniyama Jigoku (Crocodile Hell)

Oniyama Jigoku, the "Monster Mountain Hell," is the most unexpected of Beppu's seven hells. The spring itself is a fiercely hot, steaming pool - hot enough, according to local lore, to have once pulled a train's worth of steam power - but what draws visitors is what surrounds it: roughly 80 crocodiles, basking in enclosures kept perpetually warm by the geothermal heat. Opened in 1923, this was the first facility in Japan to breed crocodiles, and it has done so continuously by exploiting the free, year-round warmth that the hell provides, creating a tropical microclimate in the cool hills of Kyushu.

The pairing of scalding hot spring and cold-blooded reptiles sounds absurd, and to a degree it is - Oniyama leans into the novelty. Walkways lead past pools and pens holding various crocodile and alligator species, from hatchlings to hefty adults, and the site preserves the mounted remains of its most famous resident, a long-lived crocodile named Ichiro, as a nod to its breeding heritage. Interpretive displays explain how the constant geothermal warmth allowed successful breeding decades before modern heated enclosures existed.

Feeding times are the highlight, when otherwise motionless crocodiles lunge with startling speed; the schedule is posted at the entrance, and timing a visit to coincide with it transforms the experience. Outside feeding, the animals can be sluggish, which some visitors find underwhelming, so managing expectations helps. The steaming source spring, visible amid the enclosures, is a reminder that beneath the reptilian theme this is still a genuine hell, sharing in the national Place of Scenic Beauty designation that covers all seven.

Opinions on Oniyama are more divided than on the other hells. Animal-welfare-minded travellers may find the older-style enclosures modest by contemporary zoo standards, and those expecting the pure geothermal spectacle of Umi or Chinoike can be surprised by the emphasis on crocodiles. Approached on its own terms, though - as a century-old curiosity born from Beppu's habit of turning geothermal heat to practical use - it is a genuinely interesting stop, and a hit with children.

Oniyama is one of the five walkable Kannawa hells, sitting close to Kamado and Shiraike, so it slots easily into the on-foot leg of the Beppu Jigoku Meguri. Most visitors spend around 30 minutes here. Facilities include restrooms, a gift shop selling crocodile-themed souvenirs, and flat, accessible pathways with only minor steps.

As a piece of social history, Oniyama tells you something essential about Beppu: for a thousand years its people have looked at boiling, sulphurous, seemingly hostile ground and asked not "how do we avoid this?" but "what can we do with it?" - cooking eggs, harvesting bath salts, warming greenhouses of tropical plants, and, improbably, breeding crocodiles. Seen in that light, the Crocodile Hell is not a gimmick grafted onto a hot spring but one more chapter in Beppu's long, inventive relationship with the fire beneath its streets.