To create a mythic look to for the ancient sunken city and the Jules Verne-esque technology in Atlantis: The Lost Empire, Disney turned to Hellboy creator Mike Mignola to work on the film's production design.
While Mignola's influence can certainly be felt in the final film through his designs, Disney didn't fully copy his heavily shadowed aesthetic. We can get an idea of what the movie might have looked like if Mignola had designed it entirely from these images and from the cover of the graphic novel adaptation of the film, which was drawn by Mignola (just the cover, not the graphic novel itself). You can also see more of Mignola's concept art (and other Atlantis concept art) at Floobynooby.
Disney's Atlantis Concept Art [Floobynooby via Brian Bendis]