Tsar Bomba
The explosion had a power of 56,000,000 tonns of TNT (56 Megatons). The Tsar Bomb was actually designed as a 100 Megatonn-Bomb. But as there were problems to detonate a bomb as strong as 100,000,000 tonns of TNT, the Soviet Union technicians reduced its power to 56 Megatonns.