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1. Deep Space Habitat: Believe it or not, I consider this to be the most plausible expansion from land-based cities. Taking into consideration all the research and work already happening in outer space programs, and the fact that there already exists satellites and stations, I wouldn't put it too far beyond us to get a space habitat working and comfortably livable within 100 years.

I like this particular scenario because it can provide everything a land-based city could as well as removing the dangers of natural disasters of the planetary kind. There still are problems posed by moving objects (i.e. meteors/asteroids), radiation, and acclimation to gravity (or lack there of), as well as getting the necessary equipment, materials, and supplies into outer space in the first place.

2. Floating City: Of the three this is probably the least possible, considering the amount of power and technology it would take to keep a city afloat within an atmosphere. I'm not talking low-orbit, I'm talking just above the clouds or within the clouds, or maybe even as low as a hundred feet off the ground.

It would provide the perfect balance between land and outer space, though. It would have easier access to supplies and materials and equipment, while removing the threat of land-based disasters such as floods, tsunamis, earthquakes, and etc. It would also allow more 'freedom' to animals and forests. The danger of floating cities, however, is the fact that if they become large enough, or plentiful, they form a new layer above the earth, blocking off the Sun and killing natural resources.

3. Underwater City: My least favourite, though realistically more plausible than the other two, I would feel trapped. I'm somewhat of a closetrophobe, and the feeling of 'drowning all the time' scares me. While I wouldn't mind underwater trips every now and again, it's not very pleasant to live in.

Easy access to resources, easy access to technology make this kind of city tempting should we ever run out of land. It's easier to realize, too, since the level of technology needed is pretty close already.

I'm with Umar on this one. I would love to live in a floating city...I imagine that popular pursuits up there would be hang gliding or other sorts of flying. One of my most heartfelt dreams is that of flying, free and unfettered by machines or straps or fabric.

The city under the sea is my second choice. I agree with Ethan, I'm a bit of a claustrophobe; I get antsy in caves, imagining the weight of the rock above me. But on the other hand, I would love to be able to watch the sea creatures swimming around outside, and being able to get out there to explore the undersea environs.

Space would be my last choice. There's just something cold and antiseptic about it. And no option for escaping, if you felt like you had to. You'd be indoors all the time, and nature, real nature, isn't an option up there. I like to read sci-fi, but I'd never live there. Not by choice, anyway.

Neat! Yeah, like I said, I wouldn't mind visiting underwater, such as visiting a city, or staying in that hotel, but I wouldn't want to live there forever and ever. Unless it was a 'shallow underwater city', but nothing deep deep DEEP into the ocean.

If I must humour you, I vote for the floating city. Although I wouldn't really live there as having to live in a floating city would mean that I am not living in England. For someone who has vowed to stay here and freeze if we get another ice age, leaving is not really an option.

Yeah! Or what if the Underwater City was within England's borders?

The UAE gets all the cool hotels! They've got the tallest (soon), and now the under water one! Maaaaan!! No wonder Dubai is so popular!

Yeah, that'd be pretty cool, right? Also about the height thing, space has no perception of depth or height, that's why you prolly wouldn't be scared of heights there.