How do I make a mob that can walk through walls?

The problem with being able to go through blocks is that the ground is made of blocks, and your mob will fall to the bottom of the world.

I think that the best way to do this (probably) is to convert all nearby blocks to falling blocks without gravity, letting you go through the block, and then convert the blocks back once you're done. (I can send you a bunch of Java code if this is what you want to do.) Although this would probably confuse the AI. I have no idea how to teach the AI that it can go though walls.