I'm going to take a break from philosophy this time and offer a "zone" idea instead. Bioshock's X06 trailer with the underwater city reminded me of an old idea (most of this I'm just improvising though). Having grown up reading Jacques Cousteau books and spending whole summers on the Gulf Coast, I'm very hopeful about making use of that knowledge in design one day.
Imagine a city built into the walls of an ocean trench. It's not a unified cliff dwelling like Mesa Verde, but a collection of separate pockets of architecture scattered on both sides of the trench. The ocean shelf hangs over the top of the trench, with seaweed dangling from its edge. Beneath this smooth overhang, the trench walls are uneven and porous.
The whole city is lit with the bioluminescence of captured and concentrated plankton, in globes of various sizes and in rope-like cables. The most basic dwellings are carved from the cliff mud. The most elegant structures are covered in iridescent fish scales, pearls, ivory and abalone. Pillars of whalebone and cemented corals prevent erosion from collapsing the civilized pockets. Natural water currents are redirected and squeezed to provide quick and efficient avenues of travel between the sides of the city and elsewhere.
The pores of the trench walls, after millenia of erosion by water currents, interconnect to create long and complex cavern systems. Many of these have been explored and carefully catalogued. The majority of sizable tunnels have been gated and guarded.
All manner of creatures roam within the trench, including an intelligent race of eel-like beings with translucent bodies which make them difficult to spot until already upon their prey. As if this were not enough, countless of their kind have been exterminated to protect the ever-expanding city, and so their ghosts often haunt the city and trenchways. Thus, one is often unsure if the figure approaching from the darkness is a living eel-kin or a harmless (though frightening) spectre. It is said that in the deeper corridors, one may be caught by the undertow and dragged to the horrific dwelling of these creatures; but many say that is only a superstition.
There are other cities beneath the waves, though most are not in trenches. Some cling to floating plantlife on the water's surface, like upside-down igloos, with surface exits. These people trade with the landfolk. Another people dwells in immense coral towers and has tamed some hard-plated sea creatures as guardians and mounts. And then there are the nomadic tribes, who connect the cities in trade and provide transport in their whaleships (hollowed-out whale carcasses, kept fresh through the application of nutrients, steered through bioelectric manipulation, and divided into habitable chambers for stock and travellers).
That's all I care to come up with for now.