Detailed multibeam bathymetry data in the western Ross Sea, Antarctica, delineate a field of unusual flat-topped seafloor mounds located ∼50 km west of Franklin Island and an arcuate zone of pockmarks to the northwest and west of Franklin Island. Sixteen mounds occur in an area about 30 km square at a depth of ∼500 m, within the Terror Rift, the active extensional part of the Victoria Land Basin. The mounds tend to be circular in the east and linear in the west, with their steepest slope to the southeast, and shallowest slope to the northwest, consistent with erosion by northwest ice-sheet movement. The largest mound is ∼4 km across and 100 m high. Five similar features were delineated to the south and east of Franklin Island at depths of 400–650 m. Seismic, gravity, and magnetic data indicate that the mounds are largely low-density, nonmagnetic bodies overlying a largely nondisrupted sedimentary section, but some mounds have an associated small (∼50 nT), short-wavelength, normal or reversed magnetic anomaly, indicating a magnetic core to the mounds. Their proximity to inferred subsurface gas hydrates suggests they may be carbonate banks, but they also occur close to volcanic centers including Franklin Island. Our preferred interpretation is that they are of volcanic origin, erupted during a geomagnetic reversal and under a grounded ice sheet forming hyaloclastite edifices, previously unknown under the Ross Sea. The pockmarks range from 200 m to 500 m in diameter.