Illustration for the Books I-III Omnibus Edition, Phantom of the Earth, by Raeden Zen.
This one depicts the Theater District, Beimeni City, Phanes Territory. It's a location deep under the Earth, inhabited by transhumans, in a city that resembles a futuristic Roman empire. Although underground, the ceiling has an artificial sky-like appearance, complete with clouds and features of a real Earth sky. The ceiling is supported by giant pillars made of compressed diamond. The story is set thousands of years from our present time.