The is a geological formation that runs across the middle of Kyushu, Japan, from Beppu Bay in the east to the Shimabara Peninsula in the west. The area is known for its volcanic, geothermal, and seismic activity. It encompasses several significant geological features, including a number of active volcanoes. The most prominent is Mount Aso, an active volcanic formation consisting of one of the world's largest calderas and a central crater group with five distinct peaks. Beppu Onsen is located at the eastern end of the graben.