Recently in the North Atlantic, two nuclear submarines (this means 2 reactors and 32 nuclear-armed missiles, carrying as many as 144 nuclear warheads) tried to occupy the same space at the same time. Fortunately, there were no nuclear incidents, let alone injuries to the roughly 240 British and French sailors aboard the HMS Vanguard and Le Triomphant.

On the one hand, it is astonishing that two submarines would collide in the depths of the Atlantic. On the other, it’s surprising that it hasn’t happened before.

Both boats are designed to be as undetectable as possible. They arevery quiet, and at depth sound is all-important.

Submarines in the depths have been compared to two men stalking each other in a pitch-black football stadium, each carrying a flashlight. They can move about quietly, listening to each other, but dare not use their flashlights for fear of giving up their positions.

Submarines have two forms of sonar: passive and active. Using passive sonar is similar to listening very carefully. Active sonar is the “flashlight”: it’s making noises and listening for the echoes.

Ballistic missile submarines (“boomers”) almost never use their active sonars. Their entire purpose is to remain undetected. Indeed, the best of them are often described as “holes” in the ocean: they are actually quieter than the normal seas.

In the case of the Vanguard and the Triomphant, the two collided almost head-on. And this was actually a good thing.

First up, it explains how they collided. The most noisy part of these boats are their reactors and their propellers, situated at the subs’ rear ends. Since they were heading almost straight at each other, most of what little sounds they emitted was blocked by the rest of the boats.

Second, the bows of these boats are the most expendable parts. That is where their active sonars — their “flashlights” — are located. The Triomphant’s sonar was crushed, and the Vanguard was dented and scraped down one side.

Third, the nuclear missiles are carried amidships of these boats. The reactor is carried towards the back of the missiles. A far worse catastrophe would have seen one ramming the other in a “T-bone” collision, as that could have directly involved the missiles or the reactor.

Realistically, the chances of such a collision triggering a nuclear explosion was almost nil. The safeguards  against accidental detonation are not foolproof, but near it. The reactors that power these boats are almost as secure.

Both boats returned to their home ports under their own power (initial reports concerning the Vanguard to the contrary), and will be repaired and sent out again. Great Britain and France will probably start coordinating their nuclear missile boats’ movements a bit more closely, defining large areas where their vessels will be operating to minimize the chances of another collision.