- North Korea says it carried out its sixth nuclear test on Sunday
- It has threatened to develop nuclear-capable intercontinental missiles
Hydrogen bombs have a far larger yield than traditional weapons, meaning devices can be smaller while providing greater devastation.
Making them small enough to fit on a missile is a challenging task, and one that North Korea claims to have achieved. Experts are not so sure, but as long as Pyongyang claims to have the technology, the working assumption is that it's true.
What is a hydrogen bomb?
Hydrogen bombs, or H-bombs use fusion, the same process that powers the sun.
In a hydrogen (thermonuclear) bomb, "heavy" isotopes of hydrogen are forced together to release a much bigger punch -- hundreds or even thousands of times more powerful than the only nuclear weapons that have been used in warfare.
Atomic bombs use a process called fission. They split plutonium and/or uranium into smaller atoms in a chain reaction that releases massive amounts of energy.
Plutonium bombs tend to involve fairly large devices. Early nuclear weapons, such as the "Fat Man" device dropped on Nagasaki in 1945 -- which weighed around 4,700 kilograms and was a meter and a half in diameter -- were "much too large to place on a ballistic missile," according to weapons expert Jeffrey Lewis.
Miniaturizing any type of nuclear bomb so it can fit on top of an ICBM involves delicate balance between making the device smaller and not sacrificing eventual payload, which requires a lot of testing to ensure the smaller warhead is still capable of producing a devastating explosion.
Sixth nuclear test
North Korea's sixth nuclear test came almost one year to the day of the previous test, which was determined to have a yield of 10 kilotons.
Independent seismic monitor NORSAR estimates the latest one had a yield of as much as 120 kilotons. An official at the Korea Meteorological Administration said it was closer to 50.
Either way, it was much bigger than all previous tests and shows increasing ambition in North Korea's nuclear program.
The test follows a dramatic surge in missile testing this year. It almost certain now that the country could send a ballistic missile to other continents, many experts have said.
Pyongyang claims its intercontinental ballistic missile (ICBM) could even reach the US mainland -- but that claim needs to be treated with a degree of caution.
Can North Korea's ICBM's reach their targets?
Having an ICBM does not mean a country can deliver any payload it wants.
Designing and building an effective intercontinental ballistic missile (ICBM), one that can not only fly the required range but also have any likelihood of hitting its target, is incredibly difficult in itself.
North Korea has tested its missiles on an upward trajectory. Some experts have assumed that if it can fly a certain distance upwards, firing it at a different angle could see it reach other continents.
But it's a little more complicated than that.
An ICBM is guided by a succession of rockets, which launch the missile thousands of kilometers into the atmosphere and then around the globe, before a re-entry vehicle guides the warhead down toward its target.
Some of the missile tests have failed the re-entry process, that is, they are destroyed as they comes back down and re-enter the Earth's atmosphere to hit their targets.
"If you think of a space vehicle re-entering the Earth's atmosphere, they have heat protectors to stop it from burning up inside. The speed at which a missile comes through the atmosphere creates an enormous amount of air pressure and heat," Martin Navias from the Center for Defense Studies at Kings College London told CNN earlier.
Each stage of that process is incredibly involved and requires intense testing to get right. Even North Korea's very active missile program took almost two decades from when it first began testing long-range rockets to having an apparently functional ICBM.
Once the warhead is small enough to not affect the ICBM's range, another key question remains. Lewis asks: Can the nuclear weapon survive the shock, vibration and temperature changes associated with ballistic missile flight?
The only way to answer that question is with yet more testing, ensuring that the missile itself, the nuclear device, and the re-entry vehicle all work correctly, and are still working by the time they get to their target.