One of the most mysterious and little known aspects of the military is how the nation’s fleet of nuclear submarines work. Submarine warfare, with its emphasis on stealth and secrecy to ensure survival, tends to result in less information about how the submarine community goes about its business. But this video is a great explainer on the basics of modern submarine warfare, both how submarines operate and how subsurface sailors go about their lives, often for months at a time, in a steel tube underwater.

The most important of the American nuclear submarine fleet are the fourteen Ohio-class ballistic missile submarines. Living Underwater: How Submarines Work covers everything from why ballistic missile submarines exist—and why they are the most effective platforms for deterring nuclear attacks against the United States—to how those submarines operate and even send and receive emails while at sea. Yes, it’s cramped, it smells, and sometimes you gotta share your bed with another person. Submarine life isn’t for everyone.

The video by Wendover Productions explains how the ballistic missile submarines of an elite handful of nations conduct nuclear deterrence patrols. These submarines ensure that, in the event of nuclear attack against their homeland, their respective leadership will be able to launch a devastating, possibly civilization-ending counter strike.

It is this threat of assured retaliation, known as MAD (which stands for Mutually Assured Destruction, and also, conveniently, a synonym for insanity) that is at the core of nuclear deterrence strategy. You can’t kill millions, if not billions, of people if you’ll be dead, too, goes the idea.

At any time there are several Ohio-class submarines at sea, each carrying . Each Trident in turn carries an average of four to five W-76 thermonuclear warheads per missile, with each warhead detonating with the equivalent power of 100 kilotons of TNT. As a result, each submarine packs a combined equivalent of about 8,000 kilotons of nuclear firepower.

For comparison’s sake, the “Little Boy” atomic bomb dropped on Hiroshima that killed and wounded 150,000 people detonated with a relatively tiny 16 kilotons.

Yes, each American ballistic missile submarines carries a total weapon load that is 500 times more powerful than the bomb dropped on Hiroshima. The United States has 14 such submarines. The quick math will tell you that President Donald Trump can launch weapons with 7,000 times the force of the bomb dropped on Hiroshima. Even if, for some reason, they all were as relatively lethal and used much the same as the bomb dropped on Hiroshima, you’re looking at over a billion casualties in an instant.

And that’s just what’s on the submarines.

Unlike many navy ships, American ballistic missile submarines maintain two full sets of crews per ship, the better to maximize their time at sea. The two crews, known as blue and gold, take turns manning the ship for patrols that typically last around 77 days at a time. At the end of a patrol the sub slides into port, refuels and restocks on supplies and food, and the other crew takes over to conduct another patrol. While creature comforts are hard to come by, at least the food is pretty good, with submarines having the highest food budget per sailor of any ship in the U.S. Navy.

But, on the other hand, everyone there hopes they never have to actually do their job.