The presence of potential shorelines in the northern hemisphere of Mars suggests that Mars once had oceans. The non-constant elevations of these putative shorelines, however, do not support that hypothesis. Recent work by Robert Citron and colleagues suggests that the discrepancies in Mars’ shoreline elevation can be explained if the shorelines have been misaligned by the growth of the Tharsis volcanic province. Their model indicates that Mars’ oceans formed before or at the same time as Tharsis developed, thus providing a possible link between volcanic activity and the existence of surface water on Mars.