Immortal Cube

What is this?
This is a big cobblestone cube, it's immortal because you can't destroy it (if you don't destroy the redstone of course). 4 Cobblestone generators let the cube respawn.

How does it work?
The Cobblestone-generator generates a block, a piston pushes it away, The Cobblestone-generator generates another block, ...
when the limit of blocks a piston can push is reached, another piston pushes the blocks, then there are other pistons pushing these blocks, and pistons from below also pushing blocks.