The Tatun volcanic episode ended 350,000 years ago, but in geologic terms the landscape is still young, and the geothermal activity persists. The area's abundant rainfall trickles down into the underlying strata of quartz sandstone, an ideal aquifer which fills with hot groundwater.
大屯火山群的噴發活動雖在卅五萬年前平息，因為地質年輕，餘熱猶存；加上雨水豐沛，又有石英砂岩構成的良好儲水層，正是大量熱水形成的絕佳溫床。