History and specifics
Because nonspace had different physical laws, Megas-Tu appeared indecipherably amorphous. After Megans crafted an image of it that Humans could understand, Megas-Tu looked to be blanketed by shifting red clouds. Its surface was covered by city structures, but was entirely manipulable through magic.
In 2269, while exploring the galactic core, the starship USS Enterprise emerged from a matter-energy whirlwind into nonspace, where Lucien quickly noticed their arrival and gave the crew a quick tour of the planet. Lucien also attempted to hide the new arrivals from other Megans to avoid a public panic. (TAS episode & Log Three novelization: The Magicks of Megas-Tu)