As opposed to the atomic bomb, the kind dropped on Japan in the closing days of World War II, the hydrogen bomb, or so-called "superbomb" can be far more powerful - experts say, by 1000 times or more.
North Korea's first three nuclear tests, from 2006 to 2013, were A-bombs on roughly the same scale as the ones used on Hiroshima and Nagasaki, which together killed more than 200,000 people. Pyongyang announced that it had detonated its first H-bomb; while seismic data supported the claim of a large explosion, there was no immediate way to confirm the type.
"Think what's going on inside the sun," says Takao Takahara, professor of international politics and peace research at Meiji Gakuin University in Tokyo. "In theory, the process is potentially infinite. The amount of energy is huge."
The technology of the hydrogen bomb is more sophisticated, and once attained, it is a greater threat. They can be made small enough to fit on a head of an intercontinental missile.
"That the bomb can become compact is the characteristic, and so this means North Korea has the US in mind in making this H-bomb announcement," says Tatsujiro Suzuki, professor at the Research Centre for Nuclear Weapons Abolition at Nagasaki University.
But the H-bomb requires more technology in control and accuracy because of the greater amount of energy involved, he said. Both the A-bomb and H-bomb use radioactive material like uranium and plutonium for the explosive material.
Theoretically, this is the effect the device North Korea has tested would have if it was detonated above Auckland, as well as a 350 kiloton, the type which the US currently has in its arsenal (the following screengrabs are from NukeMap by Alex Wellerstien).
Effects of 6 kiloton airburst
• Maximum size of the nuclear fireball: If it touches the ground, the amount of radioactive fallout is significantly increased. Minimum burst height for negligible fallout: 110 metres.
• Radiation radius: 0.99km (3.1km/2)
• Thermal radiation radius (3rd degree burns): 1.2 km (4.56km/2). Third degree burns extend throughout the layers of skin, and are often painless because they destroy the pain nerves. They can cause severe scarring or disablement, and can require amputation.
Effects of 350 kiloton airburst
• Maximum size of the nuclear fireball; If it touches the ground, the amount of radioactive fallout is significantly increased. Minimum burst height for negligible fallout: 0.57km.
• Air blast: Most residential buildings collapse, injuries are universal, fatalities are widespread. Optimal height of burst to maximize this effect is 2.2km.
• Thermal radiation radius (3rd degree burns): 7.67km (185km/2).
The hydrogen bomb is in fact already the global standard for the five nations with the greatest nuclear capabilities: the US, Russia, France, the UK and China. Other nations may also either have it or may be working on it, despite a worldwide effort to contain such proliferation.
The hydrogen bomb was never dropped on any targets. It was first successfully tested in the 1950s by the US, in bombs called Mike and Bravo. Soviet tests soon followed.
Terumi Tanaka, head of Nihon Hidankyo, or the Japan Federation of A- and H-Bomb Sufferers Organizations, has been working to ban nuclear weapons for years and was stunned by reports of the H-bomb test.
"It defies hopes for progress," he said. "I am outraged."