Seen only in Legion of Lawndale Heroes, Chapter 12:6 ('Bop Til You Drop, Part I'), the Shadow Earth was an actual and complete 'virtual reality' doppelganger of the Earth extrapolated inside a VR simulation, and fashioned to be absolutely indistinguishable from the real Earth in all aspects.
The Shadow Earth was created by Malik Diamond, (the First Cadet for the USAES ACME Program), who possesses cyberpathic abilities rivaling those of Tainn Reynolds. Diamond was one of a group of Academy cadets/cyberpaths who were acting as a living adjunct for the USAES VR systems. The reason for Diamond's creation of this omni-expansion to the VR simulation was due to the actions of Quinn Morgendorffer, whose electrical powers were so great that they were actually starting to affect the world outside virtual reality. (This was because her already-formidable powers were augmented by Sherrie Klein - it would later be revealed that Sherrie had simply unlocked Quinn's ultimate potential as a Class Five metahuman - and Quinn was trying to tap into the actual Earth's electromagnetic field to intensify her blasts.)
This 'omni-expansion pack' was created by cyberpathic cadets using their powers in unison while Diamond, whose powers allow for the manipulation of and extrapolation upon nearly infinite amounds of information (in essence, a living 'system core'), gained full access to the USAES computer systems' main A.I. cores and the primary holographic activation matrix for the entire USAES VR simulation array.
In addition, Diamond was physically linked directly into the USAES primary mainframes, the USAES Academy Wiki (while also establishing a wireless connection to every electronic information-storage device with a Internet connection inside a six-mile radius), and had everything tied everything into his specially-constructed 'interlink chair' (which possesses a functional ‘neural-scan interface’ device). He then used the Magrathea system (named after the world of planet-builders in the 'Hitchhiker's Guide To The Galaxy' series by Douglas Adams to allow for instaneous modification, extrapolation and adaptations to be made within the simulation; this allowed Johnson to in effect, create an actual parallel dimension and a Earth for that dimension.
After its appearance in LLH, Brother Grimace received a polite request from Roentgen to not use the Shadow Earth again, as it was a concept that could cause problems on a number of levels storywise. This was only the second time that Roentgen has made a specific request for a major change as to how storylines should go (the first being a request to not kill off Tiffany Blum-Deckler). Brother Grimace told Roentgen that he would be happy to oblige, as he had never considered using the concept for any further fics.
The inspiration for the 'Shadow Earth came from the 'Star Trek:The Next Generation' episode 'Ship In A Bottle' and the 'Stargate Atlantis' episode 'Outcast'. Both episodes deal with the use of omni-expansion VR systems that allow the sentient programs within to unwittingly believe that they are actually living in the so-called 'real world'.