Due to the special location of Taiwan Island (in the pacific seismic belt), the volcanic movements happened frequently in the middle ancient times and resulted in various volcanic terrain, the faults, for example.  The hot springs that we see today were also formed during this period.  The water works its way down through porous rock to a depth of 1-1/2 to 2 km and was restored in the stratum, where the water was at the meantime affected by the terrestrial heat and grows rapidly in temperature as boiled water.  Once the restore layer is flawed or cracked, the hot water will be forced up along the fault and the hot spring is formed.