These volcanos went extinct about 350,000 years ago, but because that is quite recent in geological terms, a lot of heat remains. When this kind of heat is combined with ample deposits of sandstone, which are excellent holds for water, opportunities abound for water underground to warm up quickly.
大屯火山群的噴發活動在卅五萬年前平息，但因為地質年齡尚輕、餘熱猶存；再加上此區雨水豐沛，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」。