Tourists at the Aso volcano complex in Kumamoto Prefecture have had the rare opportunity to see the green crater lake in the No. 1 crater of Mount Nakadake. This marks the first time in six years that the lake has turned green. Officials confirmed the change on January 22, after observing the lake’s milky green color.
The green color appears when volcanic activity is relatively low in the area. The green crater lake gets its hue from ferrous ions, a natural occurrence when the volcanic conditions subside. This contrasts with periods of increased activity, when the lake turns milky white or cloudy gray. Volcanic ash and sand settle in the lake during these more active times, causing the color change.
By March 10, officials found that the lake’s conditions had not changed. There were no signs of swelling in the water surface, nor were there small-scale mud eruptions. The water temperature remained stable, about 60°C, and the water volume stayed at around 80% of its peak level from October 2011. This stability has persisted since late last year.
In the past, the green crater lake disappeared during more active volcanic periods. In 2019, volcanic activity led to a significant drop in water volume. The lake’s volume fell from 100% to just 30% of its peak level, turning gray before eventually disappearing. The 2021 eruption, which included a pyroclastic flow, reshaped the crater and caused the lake to reappear.
Since then, the water volume has been gradually rising, and the lake’s color has shifted back to green. Despite some difficulties in conducting surveys, officials have noted this gradual recovery. The crater lake’s surface lies about 140 meters below the crater rim, and volcanic fumes often obscure the view.
On March 10, non-Japanese tourists enjoyed the view of the green crater lake, despite the thin veil of white smoke surrounding it. The weather was favorable, and the lake’s color was visible through the mist. Visitors can expect more opportunities to see the lake in the coming months, especially during the Golden Week holidays in May, as long as volcanic activity remains low.
Beside the crater, a monument with a tanka poem written by Emperor Naruhito when he was crown prince stands. The poem reflects on the beauty of the green crater lake, saying, “I feel the vibrations/ As fumes rise from an Aso crater/ Filled with blue water.”
