Submarines: Keepers of the nuclear arsenal

Submarines are, arguably, the most important boats in the US Navy. They participate in seek-and-destroy missions, surveillance missions and "irregular warfare." Most importantly, though, some submarines hold half of the United States' thermonuclear warheads in secret locations throughout the globe.

Here's everything you need to know about these important, stealthy ships.

The 14 ballistic-missile submarines in the Ohio class are fearsome: They hold thermonuclear missiles in classified locations around the world, ready to launch at a moment's notice.

Like others in its class, the USS Kentucky (SSBN 737) here holds 24 Trident II nuclear missiles, each with up to 12 warheads. The missiles have a range of 7,000 miles.

The nukes carried by the Ohio-class submarines are meant to be used only as a second-strike response to a nuclear attack.

This lose-lose concept of "mutually assured destruction" has been a key part of US military policy since our first ballistic missile submarine was commissioned in 1959.

Here's what a launch of a real (albeit disarmed) Trident II D5 missile looks like.

This particular launch was conducted in 2016 off the Florida coast as part of the certification process for the USS Maryland (SSBN 738).

Not all Ohio-class submarines carry nuclear missiles, however -- only 14 of 18 do. That's because the US Navy converted four submarines, the USS Ohio, USS Michigan, USS Florida, and USS Georgia, into cruise missile subs armed with non-nuclear Tomahawks.

The conversion of the USS Ohio (SSGN 726), shown here in dry dock at Newport News Shipbuilding in Virginia, helped bring the US into compliance with our New START arms-limitation treaty with Russia.

The Ohio class of submarines were all built at General Dynamics Electric Boat in Groton, Connecticut.

These days, the facility is working to develop the replacement for the Ohio class, the Columbia-class submarine, under a $1.85 billion Navy contract. It is also building Virginia-class attack subs, such as the USS North Dakota (SSN 784), shown here in 2013.

Ohio-class submarines may be stealthy, but they're not exactly fast. The top reported speed of a submerged submarine is only 29 mph; subs can only travel at 14 mph while surfaced, as the USS Florida (SSGN 728) is here.

Because of their limited size, submarines can only carry a limited supply of food. And because of their slow speed, subs can't make routine port visits. That's why the US Navy operates a pair of submarine tender ships to deliver supplies and perform repairs in open water, allowing subs to go as long as 140 days without returning to port.

Here, Ohio-class submarine USS Georgia (SSGN 729) moors next to the USS Emory S. Land (AS 39) for maintenance.

Submarine tenders also serve as mobile armories for the Navy's submarines. Here, sailors aboard the tender USS Emory S. Land (AS 39) raise and secure a two-ton Mark 48 Advanced Capability (ADCAP) torpedo.

ADCAPs are carried by all US Navy subs in the Ohio class. Each of the torpedoes costs $3.5 million to build.

As part of his duties aboard submarine tender USS Frank Cable (AS 40), Navy Diver 3rd Class Eric Clark helps a fellow diver perform maintenance on the mud tank of the USS Key West (SSN 772).

The mud tank is a ballast compartment that fills with water to control the buoyancy of the sub.

Exposure to Earth's magnetic field causes submarines to build up a magnetic signature, making them easier to detect and more vulnerable to mines.

To counteract this, submarines undergo drive-in "flash deperming" treatments at magnetic silencing facilities such as this one at Beckoning Point at Joint Base Pearl Harbor-Hickam. Overhead electrical cables generate a localized magnetic field, leaving the ship in a "stable and predictable magnetic condition."

Most US submarines currently in service are members of the Los Angeles class, such as the USS Pasadena (SSN 752) seen here. According to the Naval Vessel Register, there are currently 36 of these nuclear-powered subs active.

Los Angeles-class fast attack subs come loaded with MK 67 mines, MK 60 Captor antisub mines and non-nuclear Tomahawk cruise missiles.

Each Tomahawk missile is 18 feet 3 inches long, weighs 2,900 pounds and has a range of more than 1,000 miles.

These $1.6-million missiles play a critical part in US military missions around the globe. During Operation Odyssey Dawn (2011), for example, the US fired 112 Tomahawks against more than 20 targets in Libya.

This is the control room for the USS Michigan (SSGN 727). It holds the periscope(s), navigations systems, diving controls and steering wheel. It's also home to the weapons control panel that launches the sub's deadly payload.

Most US Navy subs are equipped with traditional periscopes, such as the one being used here by Rear Adm. Bernt Grimstvedt, chief of the Royal Norwegian Navy, during a tour of the USS Miami (SSN 755).

This, meanwhile, is the upgraded control room of a newer Virginia-class attack submarine, the USS Minnesota (SSN 783). Rather than use a traditional periscope, this sub uses a sensor-laden photonics mast that takes digital images.

Construction on similiarly upgraded ballistic missile submarines is scheduled to begin in 2021. This new Columbia-class is expected to be in service from the year 2031 through 2085.

As you might expect, military submarines are incredibly cramped. Here, you can see the torpedo room weapons team on the lowest level of the USS Albany (SSN 753) preparing to receive a torpedo.

Unoccupied torpedo stows are used as overflow bunks when needed.

Here you can see the lock-out chamber aboard the USS Florida (SSGN 728). It fills with water and pressurizes, as needed, to allow Navy SEAL divers and other personnel to exit the submarine while underwater.

Leaks on a submarine are absolutely devastating. To prepare for the worst, sailors practice repairing engine-room leaks here, in the damage-control wet trainer at Naval Submarine Base New London in Connecticut.

The US Navy has submarine firefighting simulations, such as the one here at Submarine Learning Center Detachment, San Diego.

Most sailors aboard ballistic-missile submarines, meanwhile, live in tight berths like the one Secretary of the Navy Ray Mabus is touring here on the USS West Virginia (SSBN 736).

At least there are privacy curtains.

The submarine's officers, meanwhile, berth in stacks of three in a similarly cramped stateroom that doubles as an office.

There's not a lot of space in the boat's 10-by-12-foot galley, either. Still, the Navy's chefs make it work, serving up pancakes, lasagne, prime rib, pizza, enchiladas and even buffalo wings.

Meals are served here, in the submarine's mess deck. It's where the 130-member crew eat meals together, socialize and, as in this photo, host family and friends for a fun "Tiger Cruise."

If you're important enough to be a chief, you might instead have your meal here in the chief's mess. The television screens here are connected to the submarine's periscope.

Prior to April 2010, women were unable to serve on US Navy submarines. The first group of women submariners officially reported for duty in November 2011.

Women continue to make history aboard US Navy subs: On August 2, 2016, Chief Culinary Specialist Dominique Saavedra, assigned to USS Michigan (SSGN 727) and shown here, became the first female enlisted sailor to earn her submarine qualification.

Given the length of the average deployment -- typically three to six months -- sailors are understandably excited about seeing their families when they return home.

In fact, it's tradition for the first sailor to disembark from a submarine to be welcomed home with a kiss. Here, Electrician's Mate 1st Class Brian Lanham is greeted by his wife after a six-month deployment.