Perceptions have changed about how much water may have existed on early Mars. In 1970, scientists thought there may have been enough to support life - but only just. Over the next 30 years, their ideas about the planet have evolved gradually after each mission to Mars to include the presence of ever more water. If Mars really was as wet as many now think it could have been, early Martians would have been well advised to have their deep-sea diving gear to hand!