A morphotectonic depression, some 15 × 106 km2 in area and centred on the AUS-Antarctic discordance1, extends across the southern half of AUS through the south-east INDn Ocean to Wilkes Land in ATA (Fig. 1a, b). The depression is co-extensive with a negative satellite free-air gravity anomaly2 which suggests that the region is underlain by downward convecting asthenosphere. Here I outline the depression and explore its age. In a pre-breakup (>55 Myr) reconstruction (Fig. 1c), the Eastern Highlands of AUS and the Transantarctic Mountains are colinear; because the Eastern Highlands were notably high during the late Cretaceous, and shared with the Transantarctic Mountains a history of volcanism in the Jurassic, both highlands probably constituted the eastern flank of the past depression, in the centre of which thick (>8 km) late Cretaceous sediment in the Ceduna Saddle3, and a possible mirror-image in offshore ATA, mark its depocentre. A broad area of late Carboniferous marine sediments in southern AUS4 may represent the earliest sign of the depression.