Irrational’s technology director Chris Kline:

All major FPS engines (CryEngine 3, UE3, id Tech 5) are designed and optimized for static environments that the player moves through, which is a reasonable choice because, even if there are trees moving in the wind, the ground under the player’s feet isn’t going anywhere. Unfortunately for our tech team (but good for you) everything in Columbia is capable of moving.

The very ground beneath your feet could fall out of the sky at any moment, which makes for some awesome gameplay and visuals but required us to create a completely new technology that we’re calling “Floating Worlds”. You saw a little of this in the gameplay demo video (in the part right after Saltonstall jumps on the Sky-Line) and you’ll be seeing a lot more of it in the future.

More [here] on Irrational’s forums.