Minecraft wiki entry for Building_a_metropolis, associated text: Underwater: This city can be very difficult to build, especially in survival, so it's recommended for experienced builders. Build your city in a giant underwater glass dome. Another way to do this is to build lots of small domes, or otherwise normal, but waterproof houses, connected with clean-looking or transparent blocks. The food industry should be dominated by markets that sell fish from local piers, unless you export land animals and plants in the city and make farms. The entrances should be waterproof, possibly made with doors or trapdoors. It is also possible to make the entrance an underground tunnel leading to land, or a vertical tube with a staircase inside leading to the sea level. This city should be quite easy to defend against attackers, assuming you build it deep enough, as any potential attacker would need potions of water breathing just to reach it.
Unstable: The easily destroyed nature of certain blocks such as TNT or flammable blocks would make a city built out of such blocks entertaining. Backups of the world can be made so the city can be destroyed for the player's entertainment.