A gigantic earthquake with a magnitude of 9.0, the maximum ever recorded in Japan, struck the Tohoku region of Japan on 11 March, 2011. As a result, as many as 745 reservoirs in Fukushima prefecture were damaged. The failure of the earth-fill dam at the Fujinuma reservoir in Fukushima prefecture resulted in eight deaths in a village downstream. This was only the second such dam to fail completely in the recorded history of Japan, the first being Mannou Lake dam. The failure was caused by the Ansei Nankai earthquake in 1854. According to official records, of the 210,000 reservoirs in Japan, at least 20,000 dams are vulnerable to future earthquakes. Therefore, it is imperative that the failure mechanism of the Fujinuma reservoir be understood. As such, we developed several theories to explain what happened. Adherence to recommendations made in this report will reduce the potential for damage in future catastrophic events.