The third flag flew over the Cutoff Colony Theory. According to this, an early terrestrial civilization had colonized Minerva, and then declined into a Dark Age during which contact with the colony was lost. The deteriorating conditions of the Ice Age later prompted a recovery on both planets, with the difference that Minerva faced a life-or-death situation and began the struggle to regain the lost knowledge in order that a return to Earth might be made. Earth, however, was going through lean times of its own and, when the advance parties from Minerva eventually made contact, didn't react favorably to the idea of another planetful of mouths to feed. Diplomacy having failed, the Minervans set up an invasion beachhead on Luna. The Annihilator at Seltar had thus been firing at targets on Earth; the translators had been misled by identical place-names on both planets—like Boston, New York, Cambridge, and a hundred other places in the USA, many of the towns on Minerva had been named after places on Earth when the original colony was first established.

The defenders of these arguments drew heavily from the claims of the Pure Earthists to account for the absence of Lunarian relics on Earth. In addition, they produced further support from the unlikely domain of the study of fossil corals in the Pacific. It had been known for a long time that analysis of the daily growth rings of ancient fossil corals provided a measure of how many days there had been in the year at various times in the past, and from this how fast the forces of tidal friction were slowing down the rotation of the Earth about its axis. These researchers showed, for example, that the year of 350 million years ago contained about four hundred days. Ten years previously, work conducted at the Darwin Institute of Oceanography in Australia, using more refined and more accurate techniques, had revealed that the continuity from ancient to modern had not been as smooth as supposed. There was a confused period in the recent past—at about fifty thousand years before—during which the curve was discontinuous, and a comparatively abrupt lengthening in the day had occurred. Furthermore, the rate of deceleration was measurably greater after this discontinuity than it had been before. Nobody knew why this should have happened, but it seemed to indicate a period of violent climatic upheaval, as the corals had taken generations to settle down to a stable growth pattern afterward. The data seemed to indicate that widespread changes had taken place on Earth around this mysterious point in time, probably accompanied by global flooding, and all in all there could be enough behind the story to explain the complete disappearance of any record of the Lunarians' existence.