With deployments underwater typically running 90 days, life onboard a submarine is anything but normal.

Cramped quarters are the norm, and sailors must have the right technical know-how as well as determination to spend months underwater at a time.

To even be eligible to be a crewman aboard a submarine, applicants must pass a series of gruelling tests, psychological evaluations, and intensive courses.

Of course, even once qualified, serving aboard a submarine is not a walk in the park.

America has 72 submarines in the fleet, including both attack submarines and fleet ballistic missile submarines called Boomers. Sailors typically deploy for 90 days in these war machines.

On all submarines, the first task after submersion is to ensure there are no leaks anywhere onboard.

Once fully submerged, life takes on an 18 hour schedule. This schedule is divided into three six-hour segments for sleeping, keeping watch, and spending free time.

The artificial 18 hour schedule and the lack of natural light make keeping track of time difficult. According to one sailor, you 'basically know the time by what type of food you're eating. If you're eating pancakes you know it's morning and if you're eating leftovers you know it's midnight.'

Not all submarines come with enough beds. To make up for this, temporary bunks are sometimes housed in the torpedo room. Generally, sailors have 15 square feet for themselves and all their possessions.

Beds in submarines are sometimes referred to as 'coffins.'

Of course, most areas on the submarine are cramped. Hallways are absolutely no exception to that.

Bathrooms are especially minimal onboard submarines, with sometimes only one bathroom for 40 men.

Showers, too, are kept as small as possible to maximise space on the submarine for more vital components.

Submarines still make room for comparatively large kitchens. Fresh food aboard the submarine usually does not last more than a few weeks. Meals for the remainder of the deployment are cooked up from a wide variety of non-perishables.

From a relatively small amount of ingredients, chefs still manage to cook up a wide selection of favourites including lasagna.

Watch duties aboard submarines are based on both rank and specialised training. Junior personnel generally learn to stand watch at the driving sections of the submarine.

Behind those driving the submarine is the Comm section where the officer of the deck keeps watch. This is also where the submarine's periscopes are.

Close to the Comm area is the navigation section of the submarine, where the quarter master keeps watch.

Navigation is done completely electronically. The VMS screen helps ensure that the submarine is exactly where it is meant to be.

The command launch console is the ultimate home for all of the submarine's offensive weaponry.

From the command launch console, all weapons are prepared and spun up in their selective tubes. A submarine's weaponry can vary from torpedoes to Trident Ballistic Missiles depending upon the type of submarine.

Trident Missiles were developed as deterrent measures to a nuclear attack. They could travel from New York to Moscow and could destroy a city as large as Washington D.C. 12 times over. They come with both nuclear and non-nuclear warheads.

Free time is often spent in the mess halls, with televisions and a complement of around 400 movies to provide relaxation for sailors.

Sailors are also provided with cards and board games to both foster camaraderie and help pass the time.

Subs also have limited gyms -- generally small, with just one or two machines -- to allow sailors to keep in shape while deployed.

It's not easy work, but it plays a critical role in U.S. power projection due to their long-strike capabilities, ability to avoid detection, and their capability of sailing anywhere around the world.

You've seen what life is like on a submarine...

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