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Apparently this theme park opened in 1965 but closed in or around 2000. The park is located in the mountains outside of Beppu and sits beside an empty roadside.
I squeezed between a gap in a fence beside a small ticket office and then walked along a path. Most of the complex was covered in foliage but there were still a few bits and pieces of its former glory scattered around including some cool retro roller-skates.