The 1999 Chi-Chi earthquake significantly altered the landscape of central Taiwan. The most remarkable and intriguing feature is believed to be the huge rock eruption at Chiu-Fen-Erh-Shan during the earthquake, which consisted of four separate eruptions accompanied by flashes of light as seen by the residents. Here we present our analysis of the rock eruption process. We propose that a little water trapped in the interconnected fractures, which were located in the base of the rock, caused a very large amount of pressure when the water was vaporized due to frictional heat. The high vapor pressure thus generated, either triggered rock failure or enlarged cracks in the rock, which erupted simultaneously during the 1999 Chi-Chi earthquake. The four different instances of rock eruptions occurred only at Chiu-Fen-Erh-Shan. We suggest that the eruptions resulted not only from the high vapor pressure, but also from the geologic focusing of seismic waves and tectonic thrust forces together, which concentrated the energy in the region to cause the explosions during the 1999 Chi-Chi earthquake.