parts of the peninsula to subside by up to below sea level and may have caused eruptive activity to localize at one site inside the graben. Eruptions of dacitic lava began from a site slightly to the south of today's Mount Unzen and migrated north over time. The volcano rapidly grew during its first 200,000 years, forming a very large cone. Later eruptions over the following 150,000 years filled in much of the graben. Initially, activity was dominated by blocky andesitic lava and ash flows, changing to dacitic pumice flows and airfall deposits from 500,000 to 400,000 years ago. The period from 400,000 to 300,000 years ago saw the emplacement of large areas of pyroclastic flow and lahar deposits; these form the major part of the volcanic fan surrounding the volcano. Beginning 300,000 to 150,000 years ago, thick phreatomagmatic deposits were laid down, suggesting the subsidence of the volcano into its graben was rapid during this period. Activity from 150,000 years ago to the present has occurred at a number of sites around the volcanic complex, building four main domes at different times: the No-dake (70–150,000 years old), Myōken-dake (25–40,000 years old), Fugen-dake (younger than 25,000 years old) and Mayu-yama (4,000 years old) volcanic peaks. Fugen-dake has been the site of most eruptions during the past 20,000 years and lies about from the center of Shimabara. In December 1663, Fugen-dake erupted, producing lava flows that covered forest for over an extension of 1 km. In the spring of the following year, there was a flood from the Kujuku Island crater, located on the southeastern flank of Fugen-dake at an altitude of 600 m. More than 30 were killed. Unzen's deadliest eruption occurred in 1792, beginning with an earthquake in November of the previous year. On February 10, 1792, eruptions from Fugen-dake's Jigokuato crater began. Lava flows began on March 1 and continued for nearly two months. On March 22, volcanic plumes were produced, and lava also flowed out of the crater. On March 25, fumes rose and lava flowed down the northeastern part of Fugen-dake at a total length of 2.7 km. Eventually, the east flank of the Mayu-yama dome collapsed unexpectedly following a post-eruption earthquake, creating a landslide into Ariake Bay. This caused a megatsunami that reached a height of , and killed an estimated 15,000 people. it is the worst volcanic related eruption in Japan. 1990–1995 After 1792,