I've climbed inside 6 historic submarines from World War II and the Cold War. Here are the coolest things I've seen.

The USS Growler at New York City's Intrepid Museum is the only nuclear-missile submarine open to the public.

Commissioned in 1958, Growler patrolled the seas during the Cold War while carrying Regulus II sea-to-surface missiles armed with nuclear warheads.

Inside Growler, I saw one of the submarine's two missile hangars.

Each hangar could hold two Regulus I missiles, which had a range of 500 miles.

I also saw the missile checkout and guidance center, where crew members were responsible for launching Regulus missiles.

Once a top-secret area, the missile checkout and guidance center featured the necessary technology to aim and fire missiles in a process that took about 15 minutes.

The USS Cobia is docked at the Wisconsin Maritime Museum in Manitowoc, Wisconsin.

During World War II, Cobia sank 13 enemy ships, earned four battle stars, and rescued seven downed American pilots. The submarine was commissioned in 1944.

Cobia featured an upside-down broom, a Navy symbol indicating that the submarine sank an enemy ship.

Mark Becker, a volunteer at the Wisconsin Maritime Museum and a US Navy submarine veteran who served during the Cold War, told me that the broom symbolizes a "clean sweep of the enemy from the sea."

I was surprised to learn that Cobia's radio room actually works and contains the oldest operational radar in the US.

Becker said the ship's SJ-1 radar is so powerful that it can interfere with phone service in the area when it's turned on.

A torpedo launch tube in the aft torpedo room featured the submarine's namesake cobia fish from Cobia's battle flag.

Walt Disney Studios designed over 30 cartoon characters for submarine battle flags during World War II, though the USS Cobia's mascot was not among them.

Commissioned in 1954, the USS Nautilus was the world's first nuclear-powered submarine.

Submarines were previously powered by diesel engines and electric batteries that only charged while the vessel was surfaced, meaning they could only stay underwater for 12 to 48 hours at a time.

With nuclear power, Nautilus could stay submerged for up to two weeks. This allowed the submarine to complete missions such as becoming the first ship to traverse the North Pole in 1958 by sailing under the polar ice cap.

The officers' quarters featured a display of an 1892 edition of "Twenty Thousand Leagues Under the Sea" by Jules Verne.

The novel, which was gifted to Nautilus' commanding officer in 1957, features a fictional submarine also named Nautilus.

With a surface displacement of 3,533 tons, Nautilus was so large that it included the first-ever set of stairs on a submarine.

On older submarine models, ladders were used as a more space-efficient way of climbing between levels.

The stairs on Nautilus led down to the control room.

A lay services box displayed in the crew's mess provided insight into life on board the submarine.

Nautilus didn't have a Navy chaplain, so crew members took turns conducting religious services using ritual items from the lay services box.

The USS Becuna at Philadelphia's Independence Seaport Museum is the only submarine of its kind left in the world.

Becuna was commissioned in 1944 and earned four battle stars in World War II. In 1951, the submarine underwent a Greater Underwater Propulsive Power 1-A, or GUPPY 1-A, modernization. It is the only remaining GUPPY 1-A submarine in the world.

Greg Williams, the museum's manager of historic ships, showed me a battery that was enhanced in Becuna's GUPPY 1-A modernization.

As part of the GUPPY 1-A modernization, Becuna was outfitted with additional batteries, a snorkel, and other enhancements to increase its maximum speed from 9 knots to 15 knots.

Williams also allowed me to climb up into Becuna's conning tower, which is not usually included in public tours.

Located above the control room, the conning tower is where the commanding officer could usually be found issuing orders. The small, enclosed area featured Becuna's main helm, or steering wheel, periscopes, and navigation equipment.

Red light was used instead of white because it made it easier for sailors' eyes to adjust to total darkness.

Another bonus room not usually open to the public was the sonar shack, which housed the sound navigation and ranging equipment.

Accessible through a hatch in the floor, the sonar room was located under the crew's mess. Becuna used both active sonar, which sends out pings of sound that bounce off other objects, and passive sonar, which listens for other vessels.

Becuna's aft torpedo room featured re-creations of comics created by crew members, showing how sailors amused themselves on board.

The comic depicted different versions of the same crew member through the eyes of various characters, including his fellow chiefs, his supply officer, and his mother.

"This is one of your first memes, essentially," Williams said.

The USS Silversides, now docked in Muskegon, Michigan, sank 23 ships and earned 12 battle stars during World War II.

Silversides was commissioned in 1941 and was one of the most successful American submarines of World War II.

In 1942, a crew member named George Platter received an emergency appendectomy on the wardroom table, which is still on display.

The submarine was not equipped for surgery, but crew members made surgical tools from utensils and used an ironing board to prop Platter's feet up, since he was too tall to lie flat on the wardroom table. The pharmacist's mate had never performed the surgery before and kept a medical textbook open next to him during the procedure.

Amazingly, Platter survived the surgery and made a full recovery. The actual table where it all took place is on display in Silversides' wardroom, which was normally used as the officers' dining room and lounge.

Silversides also featured a soft-serve ice cream machine.

Located in the crew's mess, the soft-serve ice cream machine served as a morale booster during long tours.

Commissioned in 1944, the USS Lionfish is now a museum docked at Battleship Cove in Fall River, Massachusetts.

After sinking a Japanese submarine, destroying a schooner, and rescuing the crew of a downed American B-29 bomber during World War II, Lionfish was recommissioned during the Korean War and served as a reserve training submarine.

What I found most fascinating about visiting Lionfish was getting a close-up look at the preservation work that goes into maintaining the historic vessel.

When I visited Lionfish, it was a construction zone. Part of its deck was gated off to prevent people from falling into gaping holes in its surface as volunteers performed maintenance work.

Museum ships are largely maintained by dedicated teams of volunteers, many of whom are US Navy veterans themselves. Their work has been crucial to preserving every submarine I've visited.