In 1962, the USSR tested a hydrogen bomb which became known as the Tsar Bomba. The bomb is the most powerful device ever constructed by man. Originally designed to yield 100 megatons, it was feared the radioactive fallout would be too great and the bomb was scaled back to give a yield of 50 megatons.

The device was delivered by a modified TU-95 bomber. The airplane dropped the bomb from 10,500 meters altitude. It's fall was slowed by a large parachute to give the releasing airplane time to clear the blast area. Even so, the crew of the craft were given no assurances they would survive the test.

The bomb detonated at 4000 meters altitude. The fireball was so large it touched the ground and almost reached the altitude of the realeasing aircraft. Russian sources have always given the blast yield as 50 megatons but other sources have given the yield at 57 megetons. This is equivelent to 10 times all the explosives used in WW II including both atomic weapons used against Japan. The output generated enough energy to be equivelent to 1.4% the output of the sun.

Such a weapon was not really considered practical. The test was a show of force during the height of the cold war.

More can be read about the teste and the Tsar Bomba here: [link]