Multi-channel seismic reflection profiles from the trough areas of the southern South Korea Plateau, East Sea (Sea of Japan), reveal high-amplitude, positive-polarity bottom-simulating reflectors (BSRs) at about 500 ms two-way travel time (ca. 450 m) below the seafloor. The positive polarity and no noticeable drop of frequency (low-frequency shadow) below the BSR strongly suggest that the BSR is due to an opal-A/opal-CT phase boundary. The BSR often marks a sharp interface between low-amplitude reflections above and high-amplitude reflections below. The weak, but distinct reverse-polarity event, seen 70--150 ms two-way travel time below the BSR, may represent the base of the silica diagenetic zone. The geothermal gradients (78--111¿C/km) estimated from the sub-bottom depth of the BSR are comparable to that (98¿C/km) measured at the nearby Kita-Yamato Trough, where the opal-A/opal-CT boundary has been penetrated by the Ocean Drilling Program (ODP) drilling.