Mt. Maruyama is a lava dome that belongs to the Younger Unzen Volcano and has two peaks, namely, Shichimenzan in the north and Tenguyama in the south. It has erupted about 4,000 years ago with pyroclastic flows toward north, called "Mutsugi Pyroclastic Flows".
In the evening of 21 May 1792, due to the strong 6.4-magnitude earthquake occured just below Tenguyama, one-sixth the part of Tenguyama collapsed and generated a debris avalanche. A huge amount of clastics poured into the Ariake Sea and caused a massive tsunami that resulted in a total of 15,000 casualties in the Shimabara Peninsula as well as in Kumamoto (formerly Higo) and Amakusa areas on the opposite shore of the Ariake Sea. It is well known as "Shimabara Taihen, Higo Meiwaku" meaning "Shimabara's catastrophe, Higo's calamity"and still recorded as the worst volcanic disaster in Japanese history.