Tsar Bomba
Tsar Bomba (Царь-бомба) was, as of 2005, the largest nuclear explosive device in history. It was a detonated on October 30, 1961 as a test; this took place at a height of 4000 metres over Novaya Zemlya Island in the Arctic Sea; it was dropped from a Tu-95 bomber.
The name (Russian, Tsar-bomb) was coined in an analogy with Tsar Kolokol, an extraordinarily large bell and Tsar Cannon, an extraordinarily large howitzer. During its development the bomb was actually nicknamed Ivan.
It was a fusion bomb with a yield of ~50 megatons (the original US estimate was 57 megatons, though since 1991 all Russian sources have cited it as "only" 50 megatons [1]), though the design was capable of approximately 100 megatons (it was purposely reduced at the last minute). It was not intended for actual use in warfare, however; it was developed and tested as part of the sabre-rattling between the Soviet Union and United States in the course of the Cold War. The 50-Mt test was hot enough to have induced third degree burns at 100 km, and atmospheric irregularities caused blast damage up to 1000 km away (due to atmospheric focusing, where localized regions of destructive blast damage can be created many hundreds of kilometers away); the "dirty" 100-Mt version would have laid lethal radioactivity over an enormous area.
A bomb of this magnitude has tremendous "blowback" potential to its user, while at the same time being inefficient in radiating much of its energy out into space. Modern nuclear-weapon tactics call for multiple relatively smaller bombs to produce more damage on the ground (for example, MIRVs). It was not practical for use as a weapon in wartime, requiring a specially modified bomber that could not be used to deliver the massive bomb to a distant target.
Tsar Bomba was designed and constructed in only 14 weeks after Soviet premier Nikita Khrushchev initiated the project on July 10, 1961. The bomb itself weighed 27 tonnes and was 8 metres long by 2 metres wide; a special parachute had to be designed to allow it to be dropped from an airplane. The fabrication of this parachute required so much material that the Soviet hosiery industry was noticeably disrupted. Tsar Bomba was detonated on October 30, 1961, at a height of 4000 metres over Novaya Zemlya Island in the Arctic Sea; it was dropped from a Tu-95 bomber at 10,500 metres altitude by pilot A. E. Durnovtsev. The fireball touched the ground and reached nearly as high as the release plane and light from the detonation was visible 1000 km away; the mushroom cloud rose as high as 64 km.
The Tsar Bomba had its yield scaled down by replacing the uranium fusion tamper (which amplifies the reaction greatly) with one made of lead to eliminate fast fission by the fusion neutrons. If detonated at full yield (~100 Mt), the force of this bomb would have been approximately 6,500 times the 15-16 kiloton bomb detonated at Hiroshima and would have increased the world's total fission fallout since the invention of the atomic bomb by 25%.