The information brought back by the Apollo astronauts finally resolved the conflicting speculations concerning the Moon's nature and origins that had been mooted by generations of Earth-bound observers. Soon after the Solar System was formed, 4,500 million years ago, give or take a few, the Moon became molten to a considerable depth, possibly halfway to the center; the heat was generated by the release of gravitational energy as the Moon continued to accumulate. During the cooling that followed, the heavier, iron-bearing minerals sank toward the interior, while the less dense, aluminum-rich ones floated to the surface to form the highland crust. Continual bombardment by meteorites stirred up the mixture and complicated the process to some degree but by 4,300 million years ago the formation of the crust was virtually complete. The bombardment continued until 3,900 million years ago, by which time most of the familiar surface features already existed. From then until 3,200 million years ago, basaltic lavas flowed from the interior, induced in some places by remelting due to concentrations of radioactive heat sources below the surface, to fill in the impact basins and create the darker maria. The crust continued cooling to greater depths until molten material could no longer penetrate. Thereafter, all remained unchanging through the ages. Occasionally an additional impact crater appeared and falling dust gradually eroded the top millimeter of surface, but essentially, the Moon became a dead planet.

This history came from detailed observations and limited explorations of Nearside. Orbital observations of Farside suggested that much of the same story applied there also, and since this sequence was consistent with existing theory, nobody doubted its validity for many years after Apollo. Of course, details remained to be added, but the broad picture was convincingly clear. However, when man returned to the Moon in strength and to stay, ground exploration of Farside threw up a completely different and totally unexpected story.

Although the surface of Farside looked much the same as Nearside to the distant observer, it proved at the microscopic level to have undergone something radically different in its history. Furthermore, as bases, launch sites, communications installations, and all the other paraphernalia that accompanied man wherever he went, began proliferating on Nearside, the methodical surface coverage that this entailed produced oddities there, too.

All the experiments performed on the rock samples brought back from the eight sites explored before the mid-seventies gave consistent results supporting the orthodox theories. When the number of sites grew to thousands, by far the majority of additional data confirmed them—but some curious exceptions were noted, exceptions which seemed to indicate that some of the features on Nearside ought, rightfully, to be on Farside.