It's an abrupt shock to the ears, and it causes the eyes to widen, like a newborn who is hearing for the first time. The sound of shrimp resonates within the helmet, like a concert of a thousand castanets. On the computer screen, the latest-generation sonar paints green lines that look like tangled hair.

There are other sounds too, underwater echoes that are at once strange and curiously familiar, like footsteps on the wet floor of a bathroom. This time it's a sperm whale. Is it 10, 20 or 30 kilometers away? During the night shift on the bridge of the submarine, no one seems to notice. The animal is not a threat.

Off the coast of the French city of Brest, the French Navy begins its patrol with one of its four ballistic missile submarines, also known as SSBNs. The vessel, equipped with ballistic missiles, will patrol for 70 days. No one knows its route except the commander. "I'll dissolve into the ocean," he says. For all intents and purposes, the boat will indeed disappear. It is designed in such a way that the noise of the crew onboard is less than that of the ocean. Law prohibits identifying the people engaged in the patrol.

On board, France's active military engagements – in Afghanistan and Libya – also seem to disappear. There is no talk here about the soldiers who are dying. From the outside world, the vessel receives only good news and mundane tidbits about soccer, or the weather.

"Afghanistan or Libya are wars against terrorism. No one thinks about what we do, which is nuclear deterrence. It's the foundation for a power such as France," says one sailor who goes by the nickname torpedo boss. "It's scary, but we live in complete self-sufficiency."

The submarine, more than any other weapon, is what makes or breaks a war. Equipped with some of the most advanced technologies, the lethal ship also carries some very real weapons of mass destruction. The storage facilities can stock 16 intercontinental missiles of 50 tons each. They are far more powerful than the bomb dropped on Hiroshima. Should an order be given to fire one of those weapons, there is nothing standing between France's commander-in-chief and Commander Fabrice L. There is only a keyboard of Cold War design on which black keys stand ready to receive the launch codes.

Questions asked, and left unasked

The weapons engineer, Fabrice, has 27,000 hours of underwater experience on a submarine. This is his 19th patrol. "Of course, everyone asks the question: What if one day a missile is launched?" he says. "We're transporting a weapon that we can put to use, but the responsibility, the decision, belongs to the president of the Republic."

For the chief petty officer, Jean, that's the primary mission: "We can't ask ourselves everyday if it's a good thing or not," he says.

The first hours of the patrol are taxing. The submarine is vulnerable, and all the adjustments need to be carried out. But these 110 sailors are volunteers. They say that they were attracted by the spirit of the equipment, and the feeling of being part of an elite force. They have on average 8,000 hours of experience underwater. And they too think about the vessel's deadly potential.

"When you get to work, you have to think about the nuclear weapon, it is the weapon of last resort," says Dimitri, another chief petty officer. "We hope that we never have to execute our real duty."

At 7:40 p.m., the submarine dives. On the bridge, the orders follow one after another. "Mark 3 to the bow!" "Bring in the periscope!" "Overview of systems, ship at the ready, all systems go."

Outside, the sun is setting over the ocean. Inside, the warning siren rings out four times. A sensor provides the sound of water. "Dive beginning, dive beginning," the helmsman announces. "Fourteen meters…23 meters…35 meters." The card table tilts. On the sonar, the frigate that is escorting the ship on the surface emits a "krriiii" that is all too familiar, closely followed by its echo. The marines raise their arms in order to latch onto the bars, just like on a city subway car. Silence reigns.

"We are leaving for the great depths, far from the boats, and to pass the night calmly," the commander says, delighted.

On the diagram, the descent follows the pattern of a staircase, with levels of immersion marked at 70 meters, 200 meters, and beyond. At each step, certain weapons testing, leak controls, and exercises are planned.

A U.S. submarine is operating to the north. A Dutch one is heading for the Azores. A British helicopter carrier announces some test shots.

Around midnight, the submarine reaches its maximum depth of roughly 400 meters. After a disarmed test shot, the petty officers put the torpedoes back in place. The 10 bombs are arranged on either side of the kitchen, from which the smell of baking escapes.

France's military is careful to make sure one of the four nuclear submarines be on patrol at all times, ensuring the "protection of the sea." A second sub has to be on the ready at a moment's notice.

Recently, the commander of the force invited NATO ally ambassadors on board to show that nuclear deterrence – weapons that in Europe, only France and the United Kingdom possess – is not simply rhetoric.

Before continuing its route, the ship returns to the surface to let their visitors disembark. "The view is great," says the periscope officer. But time is short, the sailors know, and the ocean waters that await them are deep and vast.

Read the original article in French

Photo – Thomas Faivre-Duboz