Chaoshan Sag has been regarded as a prospective relic Mesozoic sedimentary sag in the South China Sea. Recent drilling confirms the existence of the Mesozoic group. In one field experiment, five sets of ocean-bottom seismometers assembled by Chinese scientists have been deployed successfully to explore the deep geological structure of the basin. The ocean-bottom seismic data show clear phases from inner crust and Moho discontinuity. The data also reveal that the crust over the Chaoshan Sag thins less compared with the neighboring areas in the northern South China Sea, and the seismic velocity of the Mesozoic formation increases considerably, which implies high consolidation of sedimentary rocks, not prone to petroleum accumulation. Such case should be avoided in future exploration of petroleum.