Onikobe's Megama and Ogama Intermittent Hot Springs

A Special Natural Monument That Has Fallen Silent

High on the southern flank of Mount Araoyama in northern Miyagi Prefecture, in the upper reaches of the Akasawa stream, two thermal vents sit roughly fifteen metres apart. They have names that translate literally as "female cauldron" and "male cauldron": Megama and Ogama. Through the Taisho and early Showa eras these were among the most celebrated natural geysers in Japan. Today, neither one erupts. Both vents have been buried by sediment and overgrown with scrub for more than four decades, yet the formal designation Megama and Ogama still carry, Special Natural Monument, the same legal category as Mount Fuji's primeval forest belt and the Japanese serow, remains in force.

This article tells the story of what these geysers were, why they were so highly valued, what happened to them, and what visitors can still experience in the surrounding Onikoube and Naruko hot-spring district.

The Setting: Inside an Ancient Caldera

The Megama and Ogama vents lie about a kilometre west-northwest of Katayama Jigoku, a steaming geothermal zone, and roughly six kilometres southwest of the Fukiage Geyser. The whole landscape sits within the Onikoube Caldera, a roughly thirteen-kilometre-wide depression spanning parts of Miyagi, Akita and Yamagata Prefectures, formed by volcanic activity some two to three hundred thousand years ago. Mount Araoyama (984 metres) is the resurgent dome inside the caldera; the surrounding ring is studded with hot springs, jigoku (literally "hells", high-temperature fumarole fields), and the steam-driven Onikoube Geothermal Power Station at Katayama. Geologically, this is one of the most active corners of the Tohoku region, and the Akasawa stream draining the south flank carries warm water down into the Aramo River system.

The two vents do not lie on the same level. Ogama, the upper one, occupies a small terrace about three metres above the stream bed; Megama opens at the water's edge on the left bank below. In their active period, the two seemed to operate almost independently of each other despite the small distance between them, a curiosity that drew researchers from the earliest days of the designation.

How the Geysers Behaved

The Agency for Cultural Affairs designation text, written when the site was added to the national register on 13 April 1933, describes Megama as an elliptical opening some sixty centimetres on its long axis and thirty on the short, normally brimming with stream water. Every fifteen to thirty minutes the hole would erupt for about three minutes, sending hot water up to roughly two and a half metres in the air and discharging close to a thousand litres each cycle. Sometimes two eruptions would follow within a minute or two of each other.

Ogama presented a different picture. The upper pool, about ten metres around the rim, bubbled quietly every ten to fifteen minutes, rippling the surface rather than throwing a column. Hot water overflowed the rim and ran down a small channel directly into Megama's vent. Older accounts, however, hold that in earlier times Ogama itself erupted to more than three metres. By the time scientific surveys began in earnest, that vigour was gone.

In the Taisho and early Showa years, these were among only a handful of naturally occurring intermittent hot springs in Japan, the other prominent example being Fukiage, just over the ridge. That rarity, combined with the dramatic Megama eruptions, is what won the site its initial Natural Monument status in 1933 and its elevation to Special Natural Monument on 29 March 1952.

Why "Special Natural Monument" Matters

Japan's Cultural Properties Protection Law recognises Natural Monuments in three categories: animals, plants, and geological-mineralogical features (which includes hot-spring phenomena and their deposits). Of these, a smaller and more distinguished subset is designated as Special Natural Monuments: features judged to hold value at a national or international level. As of 2023 only seventy-five sites in the country hold that higher status, and within the "hot springs and their deposits" subcategory the number drops to just six.

The Megama and Ogama designation rested on two arguments. First was scarcity: naturally occurring geysers are rare in Japan, and the visible eruption columns were striking. Second was scientific interest: the apparent independence of the upper and lower vents, the regularity of the Megama cycle, and the geothermal system feeding the whole Akasawa upper basin gave researchers a natural laboratory for studying intermittent thermal flow.

The Long Decline

The post-war years dealt the site a series of blows it never recovered from. The Kathleen Typhoon of 1947, the Aion Typhoon of 1948, and the Kitty Typhoon of 1949 each sent torrents of mud and gravel down the Akasawa, choking the vents. By the time the Special Natural Monument designation came through in 1952, the geysers were already buried and inactive.

Restoration was attempted, repeatedly. A first round of investigations ran in 1960, 1961, 1963 and 1973, with limited success. A more intensive emergency restoration programme followed from 1975 to 1979, and for a short while, it worked. Ogama, the upper vent, briefly returned to life, erupting to a height of up to seven metres every one to two minutes. The renewed activity lasted nine months before falling silent again in July 1979. Megama had already stopped earlier that same April. Neither vent has erupted since.

An avalanche in 1974 destroyed the maintained trail that once led down the slope from the forestry road, severing easy access. With routine monitoring no longer feasible and the surrounding hot-water alteration zone, once dotted with at least seventeen vapor vents, gradually cooling, the site has been left effectively unwatched. A 2022 field survey by Yasushi Furuta, published in the Journal of Hot Spring Sciences in 2023, located both vents but concluded that they had passed beyond dormancy into something closer to disappearance: Ogama lay under sediment and brush with only a faint ring of rounded gravel still tracing its rim, and Megama, on the stream bank itself, had been alternately eroded and reburied by every flood cycle.

Can You Visit?

In practical terms, the designated site itself is not a place ordinary travellers can reach. The trail collapsed half a century ago, and approaching the vents now requires picking a route down a steep, ungraded slope from a forestry road off Miyagi Prefectural Road 171, then walking up the stream bed. There is nothing to see at the spot, no eruption, no marked vent, only buried ground under undergrowth, and the route is genuinely difficult.

What visitors to the Onikoube area can experience, and what most travel guides actually point to when they mention "the Onikoube geysers", is a separate site about half a kilometre away in Fukiage. There, an artificially drilled facility called Onikoube Kanketsusen operates two geysers, Benten and Unryu, that erupt on a reliable schedule for paying visitors. Benten sends water some fifteen metres into the air roughly every ten minutes; Unryu reaches four to five metres at slightly longer intervals. These are not the Special Natural Monument, but they offer a vivid sense of what Megama once was, and they are open year-round.

What Else to See in the Surrounding Area

The Onikoube basin is part of the broader Naruko Hot Spring district, one of the great onsen regions of Tohoku. Five hot-spring towns make up the district, and between them they offer nine of the eleven hot-spring water types recognised in Japan, a remarkable concentration in a single area. Naruko itself, the central town, has been a healing-bath destination since at least the early ninth century, and its station-front Takinoyu bathhouse preserves an Edo-period bathhouse plan.

For striking scenery, the Naruko Gorge cuts a V-shaped chasm between Naruko and Nakayamadaira, drawing crowds in mid- to late October when the maples turn. Naruko Dam, completed in 1957, was the first concrete arch dam built entirely by Japanese engineers and forms the emerald Aramo Lake just below the Onikoube approach road. Within Onikoube itself, Mineunkaku is a long-established ryokan whose outdoor bath is fed by an actual cascading waterfall of hot spring water, an experience genuinely unlike anywhere else in Japan. Skiers come in winter to the Onikoube Ski Area, where a four-seat gondola continues running through the green season for sightseeing rides.