Britain and France say two of their nuclear-armed submarines, HMS Vanguard and Le Triomphant, collided while submerged in the Atlantic earlier this month. BBC News examines how and why this happened.
The head of the UK's Royal Navy, Admiral Sir Jonathon Band, said HMS Vanguard and the French submarine, Le Triomphant, collided at slow speed while on separate routine patrols in the Atlantic Ocean.
Both submarines were badly damaged and had to return to port for repairs: HMS Vanguard with what are described as visible dents and scrapes, Le Triomphant with a damaged sonar dome.
Do submarines have sonar to stop accidents like this?
Strategic bomber submarines are, by design, very quiet. They are designed to be as undetectable as possible because they do not want to be found.
Modern submarines do not use active sonar which involves sending out signals, only passive sonar, which listens for signals with extremely sensitive microphones.
Consequently, it would be very difficult to pick up the presence of another bomber by sonar.
How do allied submarines keep apart?
There is a scheme by which submarines from allied nations are allocated water when they are submerged.
This "deconfliction" is used by Nato as a waterspace management organisation - a sort of marine-traffic control - which enables national submarine operators to separate their submarine operations, says Stephen Saunders, a retired Navy commodore who now edits Jane's Fighting Ships.
Such systems do exist between the US and British navies, but there is no such agreement between Britain and France, which has not been a member of the NATO military structure since the 1960s.
However, while "deconfliction" would not be used when a submarine is undertaking a tactical exercise, Cmdr Saunders said it should have been possible to arrange for the vessels to be in different parts of the ocean without jeopardising operational security.
It is not clear whether this collision took place while one or both of the submarines was on a tactical mission.
Was there any risk of radiation?
The UK's Ministry of Defence says there were no radioactive leaks.
However, analysts say a major disaster could have resulted had the underwater collision ruptured the hulls, set off conventional ammunition or started a fire, although the chances of a full nuclear explosion were virtually nil.
Why have we heard about this incident so late?
The French navy issued a statement earlier this month saying its submarine, Le Triomphant, had hit a "submerged object, probably a container". The impact damaged its sonar dome and it had to return to France for repairs.
Analysts say the British MoD has a policy of not giving out any more information than it has to. After initially refusing to comment the Royal Navy admitted the collision had taken place.
HOW SUBMARINES USE SONAR
Specialised listening devices are used to detect any noise made by other vessels
Design of hull and propulsion system helps reduce 'noise'
Pulses of sound are emitted - echoes that bounce off other vessels are used to locate them
Using active sonar makes submarine more 'visible' to other submarines