I built this because I wanted to have an huge, deep shaft for another project and I wanted it to look unique and not like I just deleted a hole in MCEdit. It uses 56,448 Minecarts with TNT, since they explode on impact they will bore their way all the way to the bedrock level, I used a "Tunneler's Dream" superflat world preset, so the grass is at 237. I've included the MCEdit schematic so you can place it in your own world. It comes fully primed, all you have to do is flip the switch and watch :)

Additional Notes

Requires 1.5, since it uses Minecarts with TNT, Hoppers, and Block of Redstone