Instead, it followed a group of researchers who found themselves faced with a seemingly insuperable paradox: the discovery that an advanced human civilization had flourished in the Solar System fifty thousand years ago, despite having left no traces on Earth.

[2] These were assembled in several omnibus editions and collections: In his introduction to The Two Worlds, Hogan hinted at the possibility of a sixth book, but added that there was "nothing definite in the works";[3] there were no further additions to the series before his passing in 2010.

One of its lunar survey crews stumbles upon a desiccated, space-suited body in a small cavern on the moon, a minor mystery which rapidly becomes a major one when testing reveals that the corpse (nicknamed "Charlie") is 50,000 years old.

Those records describe it as a world being slowly strangled by an ice age, dominated by two totalitarian regimes engaged in constant warfare and sinking all its remaining civilizational energies into an attempt to evacuate its elites to the much more habitable Pleistocene Earth.

Charlie's diary includes a description of the last of these wars, which was sufficiently cataclysmic in nature to have not only destroyed Lunarian civilization, but had (via widespread use of antimatter-based energy weapons) destabilized the inner structure of Minerva so badly that the planet exploded.

They each wind up solving one half of the mystery: Hunt realizes that the Lunarians did not have reliable interplanetary capability at the time of their destruction, and had only made it as far as Minerva's moon, which they had fortified heavily due to their interminable wars.

Endowed with a vicious survival instinct by the unforgiving environment of Minerva and further whetted by their history of warfare, the Lunarians had - upon their arrival on Earth - descended into barbarism and exterminated their less-advanced, less-aggressive terrestrial cousins, the Neanderthals.

The sequel The Gentle Giants of Ganymede concerns the return to the Solar System of an ancient Ganymean spaceship, which had been trapped in a bubble of time dilation by a malfunction in its Alcubierre drive.

Earth governments react to the messages with a reversion to factionalism, with the US and USSR attempting to establish separate lines of communication with the senders in hopes of obtaining advanced technology.

However, he described the magical aspects of the Entoverse as "nonsense" and complained that the attempted rescue of Minerva was unsatisfying, adding that there was a "willingness on Hogan's part to re-activate sequences that had come to a natural halt."