In Japan, the world’s most volcanic country, there is a one percent probability of a super eruption over the next 100 years, with the possibility of catastrophic disaster. In order to predict such an eruption, precise imaging and monitoring of the massive underground magma reservoir is necessary. However, in practical terms, the carrying out of man-made earthquake probes for this purpose is almost impossible on land. We have therefore decided to target the Kikai Caldera, the one giant submarine volcano in Japan, which underwent a supereruption of 7.3 ka.