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This is a big stretch of the imagination but I think it would make for an interesting proto-dimension or even a new planet for the PCs to find via a gate.

There's a series of novels written in the 2000s by British author Philip Reeve known as the Predator Cities Quartet.
They deal with a post-apocalyptic world where various communities had to become mobile to survive and to do so, they created truly massive vehicles that are towns and cities unto themselves. Each town/city is an independent city-state with the wealthy living on the upper levels and the poor inhabiting the lower decks closer to the noise and pollution of the engines.

Because the world has been devastated, these mobile towns and cities roam about picking over the remains of ruined cities and so on... but also by hunting each other to pillage the resources the "prey" town has collected (exactly the "little fish gets eaten by the big fish" situation with animals).
An image search on the web provides a number of pictures but if you're after more information about the novels or the story-world check out the following: -

http://en.wikipedia.org/wiki/Predator_Cities
http://mortalengines.wikia.com/wiki/Mortal_Engines_Wiki
http://mortalengines.wikia.com/wiki/Traction_City

And for some inspiration (if the pictures of predator cities aren't enough), do image searches for "massive mining machines" and "NASA mobile launch platform"
We humans have made some incredibly large vehicles in the last century!

Also an idea that Frankie Boyle ran with in an old episode of "Mock The Week".

http://youtu.be/ADhcy2BiTwg

WARNING: contains swears and dark humour!