The vast area stretching from Beppu Bay through Kujyu・Aso to Shimabara Peninsula is a rift valley. Around 30,000 years ago the active volcano brought the geothermal activities that later caused two big faults on the landform. These faults and the volcanic activities of Mt. Tsurumi and Mt. Garan caused the underground magma to gush the heat upwards to make high temperatures and high-pressure puddles of water thousands of meters underground. The rainwater, once it has seeped into the soil, reaches this puddle of volcanically-heated waters and gushes out again absorbing various minerals on the way. It is said that it takes an estimated 30 years for rainwater to surface as hot springs.