Meet the Megaton Twins – the two scientists Kim Jong-un can thank for his terrifying nuclear arsenal, according to experts.
Michael Madden, who works with the 38 North watchdog, said Hong Sung-mu and Ri Hong-sop were crucial to the weapons programme's success.
With their help, North Korea went from detonating suspected duds in 2006 to its current nukes, which are up to 18 times more powerful than the Hiroshima bomb.
And with the regime's latest missile – the Hwasong-15 – theoretically able to reach Washington DC, their weapons are more threatening than ever.
Kim Jong-un can be seen addressing Ri Hong-sop while Hong Sung-mu is the man to the far left
Mr Ri was walking hand in with with Kim Jong-un with Mr Hong on the right, and Kim's wife Ri Sol-ju is behind them
Mr Hong is seen next to Kim Jong-un (centre) and Mr Ri is next to Mr Hong in Pyongyang, North Korea
Mr Madden said there were a number of people who'd contributed to North Korea's nuclear weapons over the years.
But he said Ri Hong-sop, the head of North Korea's Nuclear Weapons Institute, and Hong Sung-mu, were the 'key people' now.
He said the duo most likely got an academic grounding in the former Soviet Union, perhaps at the Yuzhnoye Design Office in Ukraine, before returning to the North.
Back home, they worked at the Yongbyon Nuclear Scientific Research Center – where Mr Hong was made chief engineer and Mr Ri, director – in the 1980s.
Now they're among North Korea's elite and were guests of honour at a performance in September celebrating the regime's nuclear scientists.
Footage shows the dictator arriving at the show hand in hand with Mr Ri, with Mr Hong at his side, while his wife, Ri Sol-ju, walks behind.
The two scientists then assume the first and second seats to his left, and when someone tries to present Kim with flowers, he passes them on to Mr Hong.
Ri Hong-sop and Hong Sung-mu are even sanctioned as individuals by the US and EU respectively, with the EU calling Mr Hong 'one of the main persons responsible' for the nuke programme.
Mr Madden said: 'They're North Korean technical experts – such people are usually members of the Korean People's Assembly and the Worker's Party of Korea Central Committee.
A photo released by the North Korean Central News Agency (KCNA), the state news agency of North Korea, shows the Hwasong-15 ballistic missile during a military parade last week
Intercontinental ballistic missiles are seen at a grand military parade celebrating the 70th founding anniversary of the Korean People's Army at the Kim Il Sung Square in Pyongyang
A number of military vehicles were being driven during the military parade to mark the 70th anniversary of the Korean People's Army
'And these guys all have job titles like 'deputy director' of the Worker's Party of Korea Munitions Industry Department.
'It can sometimes be a position with more real power than someone who's actually the director.'
Ordnance expert Yu Jin was singled out as another important figure by Mr Madden.
He said: 'He's basically related to stuff that goes boom. Even if you have a nuclear weapon, it still has to explode and so he would work on a trigger or something like that.'
Mr Madden said Yu Jin also seemed to be 'a man very comfortable in his position'.
He added: 'Based on what he wears, you can tell in his clothing, his comportment, this is a very relaxed person.
'Some of these guys look like they're about to have their morgue pictures taken and that's not the case with Yu Jin.'
Mr Madden said that while there was a lot more to North Korea's nuclear programme than the three scientists, the trio were instrumental to its success.
He said: 'These people have career experience, they have been in the programme for at least three decades if not four.
'So they were eventually going to hit on to the right thing – and that's to do with how Kim Jong-un runs North Korea.
'His father, Kim Jong-il, was a micro manager… Kim Jong-un stays out of the way. 'He does not, despite what a lot of media headlines say, punish people the same way.
'People don't get shot because a missile test fails.'