There is a book named "15 Minutes:General Curtis LeMay and the Countdown to Nuclear Annihilation" by L Douglas Keeney which discusses this subject and SAC related issues. It discusses how Edward Teller proposed a ten gigaton bomb but the Air Force never funded the project. It would be the ultimate doomsday planet killer.It would probably crack the crust of the Earth.How would you deliver such bomb in that magnitude? Maybe there is a design in existence but its very classified.