In 1792, Mt. Mayuyama standing on the west side of Shimabara City collapsed due to large earthquakes frequently occurred in the last stage of the Mt. Unzen-Fugendake eruptions. The collapsed earth and sand turned into a debris avalanche, and not just filled the southern part of the town of Shimabara, but also plunged into the Ariake Sea, generating a massive tsunami. This tsunami swept into the country of Higo (now Kumamoto) on the other side of the Ariake Sea, and caused an extensive damage both in the Shimabara Peninsula and the Kumamoto area. The number of victims reached 15,000, and this disaster has been known as the largest volcanic disaster in Japanese history.
From the observation deck in Nita Park, you can see vivid details of the collapsed walls of Mt. Mayuyama on the mountain side, and look across the broad panorama of countless "hummocky hills", created by the debris avalanche that flowed down to the city area. Around the Chichibugaura Park, the interior of hummocky hills can be clearly observed due to sea erosion.