It's the French Navy's most secret base. And with good reason: Ile Longue, in the harbor of Brest, Brittany, is home to the naval component of France's nuclear deterrent: Four nuclear-powered ballistic missile submarines (SSBN), each capable of carrying 16 nuclear missiles – a thousand times the power of the Hiroshima bomb. Four "black boats," as they are nicknamed, of which there has always been at least one patrol non-stop since 1972, according to the principle of "permanence at sea." Their role: To disappear into the ocean to be in a position to deliver nuclear fire wherever they may be ordered to do so by the French president.

Land, sea and drone patrols ensure the base's security. More than 2,000 employees are required to show their credentials when entering the base. Scanners, facial recognition systems and dog teams keep a watchful eye on security, as Le Monde saw during a visit in December 2024. Cell phones and other electronic devices are prohibited throughout a large part of the base and must be stored in special cases at multiple checkpoints.

Yet this has not prevented highly sensitive information from leaking out of this security fortress. Our investigation reveals that submariners inadvertently exposed information online about the pace of SSBN patrols through their use of the Strava app, on which they can record and share workouts. The revelations follow on from Le Monde's StravaLeaks investigation, in the autumn of 2024. Bodyguards for the French, American and Russian presidents, who were also users of the app, could be tracked and the presidents' trips anticipated through their guards' Strava accounts.

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