This is the chilling way Trident nuclear submarine commanders would know that the UK has been destroyed in an attack
The crews of our nuclear deterrents have a chilling way of knowing when their deadly payload will need to be launched
RIGHT now, a British Trident nuclear submarine is gliding deep beneath the waves, primed and ready to launch a devastating strike if the worst should ever happen.
Our nuclear subs operate in the depths of the ocean, where nobody can detect them, and are always on alert to respond to the threat of a nuclear war.
Trident sub commanders keep in contact with the surface as much as possible, but they're relatively cut off from Britain when on deployment.
In the event of a sudden and unexpected strike, the sub crews may have no way of knowing exactly what's happening back home.
But whenever they fear the worst, or need to check in on us, it's thought that they tune in to Radio 4.
The radio station is the UK's most reliable, and is the one which is used as the national broadcaster in times of war or disaster.
Even if every other radio station has been shut down, Radio 4 will continue to broadcast right up to the bitter end.
This has led to the claim that the station acts as a means of checking whether everything is still hunky dory back home.
All of the submarines are famously issued with a "letter of last resort", written by the prime minister and detailing what to do if Britain is devastated by a nuclear attack.
If sub commanders can't contact anyone through their usual channels, and if Radio 4 isn't broadcasting for a set amount of time, then they have no choice but to assume the worst and open their letter.
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Trident crews never read the letters, but a sub commander faced a scare in 2004 when Radio 4's Today Programme mysteriously went off air for 15 minutes.
Caused by a fire alarm at BBC HQ, the radio silence put our patrolling nuclear submarine on alert, even though the downtime only lasted for a few minutes.
The Ministry of Defence has an understandable policy of keeping hush about Trident protocol.
However, the usual reliability of Radio 4 means it's a good way to test whether Britain has been attacked.
On that cheery note, we recently told how one estate agent had released a bizarre guide detailing where in the UK you can buy a home to survive a nuclear attack.
We also recently revealed the chilling parallels between the events of the cold war and the events of 2017.