The H-bomb, or "fusion" bomb, is one of the most dangerous weapons available on the planet.
It is much more destructive than a "fission" (atomic bomb) because it combines the power of the atomic weapon with extreme heat, under intense pressure. The two-step process uses a fission bomb as a trigger for the second stage in which atoms are driven to bind together.
The environment produces a reaction that releases much more power than the atomic bomb and therefore can do more damage.
Damage easy to deliver
The reaction for a hydrogen bomb can occur within small, light-weight vessels, which means that damage is easy to deliver.
The US began developing thermonuclear weapons (hydrogen bombs) in the 1950s after the former Soviet Union developed its atomic defence capabilities.
It's nuclear cousin, the atomic bomb, is a fission bomb that splits the nucleus within atoms.
Heavy materials such as uranium or plutonium are used for maximum impact.
The bomb works by firing neutrons at the nucleus in the uranium or plutonium atoms, which separates them into smaller and lighter nuclei.
The energy released causes these nuclei to keep splitting within the atom.
These weapons were developed during World War II. US aircraft dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki in 1945, leading to hundreds of thousands of Japanese deaths and bringing a quick end to World War II.
Nuclear weapons can be boosted, which means titanium gas is injected into a core of uranium or plutonium. When the fission bomb triggers the treated atom, the power of the weapon is multiplied.
Some analysts believe North Korea's recent test involved boosting.
How to detect hydrogen bomb testing:
To verify Pyongyang's claims, scientists can analyse underground changes in earth and water to look for signs of an explosion. They also look for particles in the atmosphere that could point to radioactive activity and use infrared monitoring to measure any atmospheric changes.
