Modern nuclear submarines are so well cloaked that in 2009, two French and British nuclear ballistic missle subs collided in the atlantic ocean by pure chance. Moving very slowly, they were’t able to detect each other just feet apart.

HMS Vanguard and Le Triomphant submarine collision

Nuclear submarine HMS Vanguard arrives back at HM Naval Base Clyde, Faslane, Scotland following a patrol.

A Triomphant-class submarine (here, Vigilant)

The submarines HMS Vanguard and Triomphant collided in the Atlantic Ocean in the night between 3–4 February 2009. Both are nuclear-powered ballistic missile submarines. The Royal Navy’s HMS Vanguard and the French Navy’s Triomphant both sustained damage, but no injuries or radioactivity releases were reported. At the time of the collision, both vessels were submerged and, according to the UK Ministry of Defence, moving “at very low speed”; both are equipped with active and passive sonar, although only the latter is used on an operational patrol.

Submarines

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Source: https://en.wikipedia.org/wiki/HMS_Vanguard_and_Le_Triomphant_submarine_collision