The 22,000lb or 10 Tonne Grandslam Bomb was a monster used back in world war 2 by the RAF. Invented by Barnes Wallace bouncing bomb creator, the Grand Slam was actually a bunker buster before the term ever existed. The idea of the bomb was to drop it from maximum altitude and allow gravity to accelerate it to trans-sonic speeds, upon impact with the ground it would bury itself 30+ feet into the surface and after a short delay detonate. The subsurface blast would cause a localised earthquake destroying all structures still standing after the initial blast. None of the German bunkers were safe from this weapon.
The MOAB I'm afraid to say is a little lighter than this device and so despite claims to the contrary is not the largest bomb in the world.

Worthy of note is the bloody good bomb aiming in the video, remember these things were aimed with a MK1 eyeball and a cross-hair!

Enjoy.