When the Viking spacecraft arrived over Mars in the 1970's they saw what appeared to be the ancient shorelines of a long dead Martian ocean, but this theory was largely discredited bv the later Mars Global Surveyor mission of the 1990's. The far more sensitive Mars Global Surveyor imaged the surface to a resolution of a few hundred metres, and that seemed to prove that the opposite shores of the "ocean" varied in elevation by several kilometres, making for a very unusual ocean indeed, in fact an ocean that simply could not have existed. Until now this is the view that has prevailed, but scientists at the University of California, Berkeley, the University of Toronto and the Carnegie Institution in Washington have now re-accessed the data, and their startling conclusion once again puts an ocean on Mars, and a very deep one at that. The scientists have changed their mind because a new theory suggests that Mars has undergone some major upheavals, at the heart of which is an exotic event called true polar wander, a process in which the very poles of the planet shifted, resulting in massive deformation of the surface. If this is the case, then those improbable shorelines may indeed be the edges of an ancient ocean, one that once covered an entire hemisphere of the planet to a depth of several kilometres. If that is the case, then the next question to answer is, where did all the water go? The Independent Newspaper has an excellent indepth article on the new theory.