By Gordon Corera
BBC security correspondent
Pakistan's political crisis has raised a number of troubling questions - could the state collapse, could militants come to power, could the country's nuclear arms fall into the wrong hands?
Anxiety about the weapons has been particularly intense in the US.
One opinion article in the New York Times talked about the need to explore military options to secure the weapons.
The subject is even up for debate in the US presidential campaign, where Hillary Clinton said she would try to get Pakistan to share the responsibility for the weapons with a delegation from the US.
But is there a real threat or is it simply a case of media hype and scaremongering?
For Retired Brigadier General Naeem Salik, who worked in the National Command Authority, the heart of Pakistan's nuclear establishment, it is a case of distrust rather than real danger.
"It appears people are very reluctant still to accept Pakistan as a nuclear power," he argues.
Post 9/11 fears
But others say it is the history of Pakistan's nuclear programme that has created much of the concern.
After 9/11, the US became fixated on the idea of terrorists getting their hands on nuclear weapons.
That fear was heightened when it was revealed that two former members of the Pakistani nuclear establishment had travelled to Afghanistan to meet Osama Bin Laden.
And then in 2004 it emerged that AQ Khan - the man known in Pakistan as the father of the bomb - had sold nuclear secrets, including even the design for a nuclear bomb, to a virtual gallery of rogue states in the eyes of Washington - including Iran, Libya and North Korea.
Concerns about a repeat of AQ Khan's actions has diminished
But it is the current instability which has aroused most concern.
The real worry is not the selling of technology to other states but rather the weapons falling into the hands of extremists within the country.
"The plausible scenario is the one in which some elements of the Pakistani military have been infiltrated by the extremist Islamic forces, probably connected with al-Qaeda," argues Bruce Riedel, who worked on Pakistan for three decades at the CIA, "and [they] steal one or two bombs, just take them off the shelf for al-Qaeda to use somewhere in the world".
But others are less sure that this scenario is realistic, partly because of the safeguards Pakistan has put in place.
As with most countries, these are kept secret but, because of the growing concern, Pakistan has begun to reveal some of the measures it takes:
- The weapons are kept in parts, with the fissile material and the delivery system (the missile) separate from the rest of the weapon
- The exact location of those facilities is kept secret and they are well guarded by a Strategic Forces Command consisting of thousands of soldiers
- The weapons themselves can only be launched by someone who has access to electronic codes
These codes are a Pakistani version of Permissive Action Links (PALs), used by the US and other countries.
"Pakistan has developed its own PAL systems which obviously ensures that even if an unauthorised person gets hold of a weapon he cannot activate it unless he also has access to electronic codes," explains retired Brig Gen Naeem Salid.
"So that is one thing, the second one is that usual practice, which is there in all nuclear weapons states, which is the two-man rule. There is a need for authentication at every level and authentication by a second person."
These measures mean fears have focused not so much on a break-in and the unauthorised theft of a bomb but on the insider threat - either coming from a small cell of sympathisers or from a coup led by extremist elements within the military.
Even here, officials says measures are in place to watch for such dangers.
America does not provide direct technological help but it has provided other forms of assistance including help in instituting a personnel reliability programme to vet staff.
Protecting the weapons is one of the few issues that an often deeply divided country can unite around
In the days of AQ Khan, security around the nuclear programme was designed merely to protect the programme against outside threats but now the systems are designed to watch over staff and their activities as well - including any signs of growing sympathy to extremist views.
But if the worst was to happen and there was an extremist coup or state collapse, what could the US actually do to prevent the nuclear weapons falling into the wrong hands?
Some say the US must be prepared. Mike O'Hanlon from the Brookings Institution wrote a widely discussed article in the New York Times looking at the options, but he argues that the US military could only intervene to secure the weapons with the assistance and permission of at least part of the Pakistani state, something which remains unlikely.
'No good options'
One problem for the US is knowing where the weapons are.
Pakistan does not allow any access to them precisely because it fears the knowledge being used to target the weapons (the Pakistanis also refuse certain types of technical help for fear that the US will install tracking devices to locate the weapons or backdoor methods of disabling them).
"I don't think there are any good options," says Bruce Riedel.
"First of all if a couple of weapons or even just one were stolen out of Pakistani's inventory by extremists we might even not know about it until it's too late.
"I think the odds are that if they are able to successfully infiltrate the Pakistani security apparatus and take one of those weapons we are not going to know anything about it until the mushroom cloud has exploded."
Nuclear weapons are often referred to as the crown jewels by Pakistanis.
Amidst all the difficulties of recent decades, they are a symbol of what the country can achieve, and protecting the weapons is one of the few issues that an often deeply divided country can unite around.
One recent Pakistani editorial argued that the real issue was not to protect Pakistan's weapons from extremists but from the evil designs of the US who were planning operations to remove them.
And so the current, heated debate perhaps tells us as much about the complex relationship between the US and Pakistan as it does about nuclear security, with both countries - ostensibly allies - remaining suspicious of each other.
Securing Pakistan's Bomb can be heard on the BBC World Service from 3 February.