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The train from Kumamoto changes direction twice as it zigzags up the formidable wall of the Aso Caldera. This ancient crater, measuring 18km from east to west, 24km north to south and over 120km in circumference, was formed about 100,000 years ago when a vast volcano collapsed. As the rock cooled, a lake formed, but the eruptions continued, pushing up five smaller cones, today known collectively as Aso-san (阿蘇山). At the eastern end of the chain lies the distinctively craggy Neko-dake (1433m), while the next peak west is Taka-dake (1592m), the highest of the five summits, and its volcanic offshoot Naka-dake (1506m). West of here lie Eboshi-dake (1337m) and Kijima-dake (1321m). Of the five, only Naka-dake is still active; it’s really just a gash on the side of Taka-dake, formed by a volcanic explosion which created a secondary peak. Naka-dake’s most recent eruptions occurred in the early 1990s, since when it has calmed down considerably, but it’s wise to treat the mountain with respect. Notices are posted in the train and bus stations when Naka-dake is closed, but if you plan to do any long-distance walks around the crater it’s wise to check at the information office. Anyone suffering from asthma or other respiratory problems is advised not to approach the crater rim because of strong sulphur emissions.

Seven daily buses shuttle visitors from the terminal outside Aso Station on a dramatic forty-minute journey up towards the peaks of Aso-san. As the road climbs up to the pass between Kijima-dake and Eboshi-dake, you look down on the perfect cone of Komezuka, the “hill of rice” – its dimpled top is said to have been created when Takeiwatatsu-no-mikoto scooped up a handful of rice to feed his starving people. Turning the other way, you get your first glimpse of Naka-dake’s gaping mouth across the grassy bowl of Kusasenri plateau, speckled with shallow crater lakes.

On the plateau, the bus stops outside the missable Aso Volcano Museum (阿蘇火山博物館), though you might want to get off here to climb Kijima-dake (杵島岳; 1321m), which rises behind the museum. The paved path from the far northeast corner of the car park takes you on an easy thirty-minute climb, rewarded with more views over the caldera, and then down into Kijima-dake’s extinct crater. From here, you can descend via a ski slope to join a path alongside the road to Naka-dake; the whole walk should take under ninety minutes.

Rather than backtracking, you could take a great hiking trail round the crater’s southern rim to the summit of Naka-dake, followed by a possible side trip to Taka-dake (高岳) and then down to the northeastern Sansuikyō Ropeway. It’s not too difficult as long as you’ve got good boots, plenty of water and you keep well away from the edge. To pick up the path, follow the boardwalks heading south round the crater across the Sunasenri plateau.