The last Sea is a typical bark-arc basin consisting of basins, plateaus, ridges, and seamounts. The Ulleng Basin, located in the southwestern corner of the last Sea, contains thick Neogene sedimentary sequence. Analysis of over 2,500 km of single-channel seismic reflection data suggests that hemipelagic sedimentation prevailed over much of the basin during the late Miocene and pelagic sedimentation became more dominant during the Pliocene. During the Pleistocene terrigeneous sediments transported by turbidity currents and other gravity flows, together with continuous hemipelagic settling, resulted in well-stratified sedimentary layers. Influx of terrigenous sediments during the Pleistocene formed depocenters in the western and southern parts of the basins. In the Ulleung Interplain Gap, where the Ulleung Basin joins the deeper Japan Basin, sediment waves suggesting bottom current activities are seen.