The Nautilus, The Oceans of the World
Captain Nemo's motto is "Mobilis In Mobile," that is, "Mobile in the Mobile Element." So we've got a kind of literary nesting doll situation on our hands here. The mobile element = water, and the Nautilus is a vessel that's mobile in this water. But the Nautilus is so fast and maneuverable, it's sort of like water itself; it's always moving, always shifting.
You can think of the setting in 20,000 Leagues in the same way. There are two very mysterious environments to be explored, one within the other: the Nautilus within the world's great seas.
Though the Nautilus may seem insignificant compared to the oceans it travels through, it's almost as amazing and mystifying. Sure, it's hard to beat the lost city of Atlantis on our Amaze-O-Meter, but this submarine boasts futuristic technology that scientists like Aronnax drool over. It's powered by Nemo's "special electricity," and, in truth, it holds just as many secrets as the oceans it explores.
In the end, the physical setting in 20,000 Leagues is sort of like a riddle, wrapped in a mystery, inside an enigma (to quote everyone's favorite cigar-chomping Prime Minister, Winston Churchill). Verne gives his characters a lot of room to flirt with mystery in these nested worlds. And at the center of it all is our favorite riddle of a captain, Mr. Nemo.