The United States tested a hydrogen bomb at Bikini Atoll in 1954 that was over 1,000 times more powerful than the atomic bomb dropped on Hiroshima in 1945. Britain, China, France and Russia have also created hydrogen bombs.
What would a successful hydrogen test mean?
North Korea claimed that it successfully staged a hydrogen bomb test in January 2016, but experts were skeptical.
A successful test this time would show that the North’s nuclear program has become more sophisticated and that the country is closer to making an atomic warhead that could be fitted on a long-range missile able to strike the mainland United States.
The underground blast, which caused tremors felt in South Korea and China, was the first by the North to surpass the destructive power of the bombs dropped on Hiroshima and Nagasaki.
If the North has the capability to build a hydrogen bomb, it could open the way to making warheads that pack much more destructive power in a smaller space. It could also enable North Korea to enhance the threat from its limited stocks of enriched uranium.
What will experts look for?
Analysts who advise governments on nuclear weapons will study the shock waves from the blast measured by monitoring stations. They will also look for clues from traces of nuclear gases that could float into the atmosphere.
Those traces may tell if this test was really a hydrogen bomb, or perhaps something less than a full-scale thermonuclear device. But it can take weeks for the gases to leak out and be detected.