The first hydrogen bomb, tested in November 1952 by the United States, yielded energy equivalent to 10 megatons of TNT - roughly 1,000 times larger than the atomic bomb dropped on Hiroshima seven years earlier.
In this new type of bomb, two types of hydrogen - deuterium and tritium - are fused into helium, thereby releasing energy.
A conventional nuclear weapon uses fission - splitting rather than fusing the nuclei of atoms - to create a less powerful explosion.
A hydrogen bomb uses an initial fission reaction to produce the high temperatures required for the more powerful fusion reaction.
Less than a year after the US thermonuclear bomb test, the Soviet Union exploded their own H-bomb, starting a race to produce more "superbombs."
The most powerful H-bomb ever detonated - and therefore the biggest ever man-made explosion - was used by the Soviets in 1961. Known as the Tsar Bomba, it produced a yield equivalent to 50 megatons of TNT.