Around 140 crew onboard a Royal Navy Trident nuclear submarine almost died after almost plunging into a 'danger zone'.

The Vanguard-class vessel was plummeting towards its crush-depth but was salvaged just before tragedy struck. The depth gauge - which tells sailors how far below the waves they are - failed as it carried 140 members of crew and Trident 2 missiles through the Atlantic.

If the sub, which is patrolling the world's waters to strike back in the event of a nuclear attack on the UK - had reached the depth, it would have reportedly sparked carnage and the chaos of a salvage mission to recover the top-secret vessel and its nuclear reactor.

The Mirror reports the submarine was about to go on a patrol when the dials stopped working. Commanders thought the ship was level when it was in fact still diving towards the sea floor. Thankfully, engineers at the other side of the 500ft vessel noticed a second dial and raised the alarm as it plummeted towards the "danger zone".

Speaking to The Sun, a source said: “It’s not the engineers’ job to control the sub’s depth but they saw how deep they were and realised something was wrong. Technically the sub was still at a depth where we know it can operate, but if it ever has to go that deep the whole crew is piped to action-stations. That hadn’t happened. The sub wasn’t supposed to be there, and it was still diving. And if it had carried on going, it doesn’t really bear thinking about.”

The sub or the depths involved have not been named for security reasons. But the fact that the disaster was saved indicates that safety systems aboard the Vanguard-class vessel are functioning properly. An urgent safety probe has been launched by the Naval top brass, but sources say the near-miss did not interrupt the UK's nuclear deterrent.

Since 1969, at least one Royal Navy submarine has been patrolling the world's waters to strike back should a doomsday attack be launched against UK interests or the British mainland. At present, the UK has four Vanguard-class submarines. Only two of them are operational as one is being renovated and the other is on sea trials. The latter ship recently had repairs that cost £300million over budget.

The Navy said: “We do not comment on operations. Our submarines continue to be deployed globally, protecting national interests.”

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