Update: Though it became possible to view the Aso crater for a few days, it closed again soon after. Thanks to Jennifer for pointing this out. The current status of the crater can be found here:

http://www.aso.ne.jp/~volcano/info/

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This is old news but, in case you missed other announcements we made about it, the volcano has reopened. – Kirk

https://www.japantimes.co.jp/news/2018/02/28/national/tourism-mount-asos-crater-area-resumes-volcanic-activity-diminishes/#.Wq0Gn2aB2u4