Antarctica has traditionally been considered continental inside the coastline of ice and bedrock since Press and Dewart (1959). Sixty years later, we reconsider the conventional extent of this sixth continent. Geochemical observations show that subduction was active along the whole western coast of West Antarctica until the mid-Cretaceous after which it gradually ceased towards the tip of the Antarctic Peninsula. We propose that the entire West Antarctica formed as a back-arc basin system flanked by a volcanic arc, similar to e.g. the Japan Sea, instead of a continental rift system as conventionally interpreted. Globally, the fundamental difference between oceanic and continental lithosphere is reflected in hypsometry, largely controlled by lithosphere buoyancy. The equivalent hypsometry in West Antarctica (-580 +/- 335 m on average, extending down to -1.6 km) is much deeper than in any continent, but corresponds to back-arc basins and oceans proper. This first order observation questions the conventional interpretation of West Antarctica as continental, since even continental shelves do not extend deeper than -200 m in equivalent hypsometry.