Architect Eugene Tsui is walking the line between the two with his two-mile-high Ultima Tower concept.

Blogger Mahesh Basantani comments:

We’ve seen a whole slew of gigantic, volcano shaped, city-in-a-building towers, each promising to be the largest building in the world. First it was the wacky X-Seed design for Tokyo, and then even Norman Foster got into the game with his proposal for the massive ‘Crystal Island’ development in Moscow. Well now, architect Eugene Tsui is taking the gigantic volcano tower concept to a whole new eco level, by taking design inspiration from the natural world. His new design for the Ultima Tower – a 2-mile high Mt Doom-esque structure - borrows design principles from trees and other living ystem to reduce its energy footprint. We are always intrigued by architecture that uses biomimicry – the borrowing of principles from nature’s designs - and Tsui’s concept for this towering, ultra-dense urban development has certainly captured our attention with its thought-provoking design.

The base of the tower is 7000 feet in diameter. Solar panels on the outside would provide a good deal of the the required energy for operation. Additional power would come from wind turbines. Plus, Tsui describes a method of generating energy based on temperature differential between the bottom and the top of the tower. (I'me familiar with the idea of generating power using the temperature differentialin water; I suppose air would work the same. But if there's that much difference in temperature between the top and the bottom of this thing, either the top or the bottom -- I'm guessing the top -- would be pretty uncomfortable.) The 144 elevators would be powered by compressed air.

In addition to trees, one of the inspirations from the natural world for this design was an African termite mound:

Tsui's design calls for the tower to be surrounded by lakes on all side, which got me wondering -- why not ocean? Could something like this be constructed out at sea, attached to a huge platform bolted to the sea floor? It would be like the ultimate oil rig. If so, I think a design like this would be a good endgame for the Seastedders -- folks who want to, in their own words, "create permanent dwellings on the ocean - homesteading the high seas."

Their concept photo looks a lot less ambitious than the Ultima Tower:

On the other hand, it looks like it's designed for -- among other things -- agriculture. That would probably be an important consideration when building a self-contained habitat out at sea. Ultima Tower would be a huge undertaking even on land. But I love the idea of having it sit out somewhere in the middle of the Pacific -- an independent city-state. There's something very appealing about that.