I toured the USS Becuna, a World War II battleship and the only submarine of its kind left in the world. Take a look inside.

The USS Becuna completed five patrols, sank 3,888 tons of shipping, and earned four combat stars in World War II.

After the war, the Becuna served as a training ship for students at the Naval Submarine School in Groton, Connecticut, while occasionally carrying out surveillance missions against the Soviet Union and visiting foreign ports.

In 1951, the Becuna underwent a Greater Underwater Propulsive Power (GUPPY) 1-A conversion. The submarine was outfitted with additional batteries, a snorkel, and other modernizations to improve its submerged speed and endurance.

The Becuna is now the only GUPPY 1-A submarine left in the world.

I visited the Independence Seaport Museum in Philadelphia, where the Becuna is docked alongside the Olympia, the oldest steel warship still afloat in the world.

The Olympia's service began in 1895. The warship was decommissioned in 1922 and became a museum in 1957.

The Becuna opened to the public as a museum exhibit in 1976.

The Delaware River's fresh water is less corrosive than salty seawater, which helps preserve the historic ships.

After I descended a steep set of steps, the first stop in the Becuna was the forward torpedo room.

Torpedomen slept in pull-out bunks, ready to load and fire the torpedoes at a moment's notice.

The forward torpedo room had six torpedo tubes, while the aft torpedo room in the back of the boat had four tubes.

The forward torpedo room also featured the escape trunk, an emergency exit for crew members.

The escape trunk only worked if the submarine was submerged in 100 feet of water or less.

"The average depth of the world's oceans is 2.3 miles, so the odds of you actually being in 100 feet of water are not great," Williams said. "This was more of a morale booster."

The officers' area of the submarine featured a ward room, a pantry, a shower, and their quarters.

The Becuna had a total crew of 80 men: eight officers, five chief petty officers, and 67 regular crew members.

In the hallway, Williams opened a hatch in the floor to show me one of the batteries that was enhanced during the GUPPY 1-A modernization.

After its GUPPY 1-A makeover, the Becuna's max speed increased from 9 knots to 15 knots.

Officers ate their meals and talked strategy in the ward room.

The ward room functioned as an operating theater in medical emergencies and was also upgraded to serve as a backup navigation center with the necessary equipment built into the table.

The fake lobster on an officer's plate is part of the museum's strategy to keep younger visitors engaged on the tour. Those who spot all of the lobsters hidden throughout the submarine win a prize.

The officers' meals were reheated and plated in the officers' pantry.

The officers ate the same meals as the rest of the crew, but enjoyed a more elevated dining experience in the ward room.

The officers had their own shower, but no one on the USS Becuna bathed very often.

Showers were infrequent aboard submarines to preserve fresh water. The space was often used to store items like potatoes, onions, and cans of food, Williams said.

The junior officers' quarters featured four bunks with curtains for privacy.

Junior and senior officers served in administrative roles.

The three bunks in the senior officers' quarters also included curtains.

The executive officer, who was second-in-command under the commanding officer, slept here.

The commanding officer was the only person on the submarine who got his own room.

People often colloquially refer to the person in charge of the ship as the captain, but his official title was commanding officer.

"In the US Navy, 'captain' is a rank, not a position," Williams said. "What most people would think of as a captain is the commanding officer, but his rank is probably going to be commander or lieutenant commander."

Five chief petty officers, who interfaced with the crew to carry out officers' orders, had their own quarters.

"There's a saying in the Navy that officers command but chiefs make the boat go," Williams said.

All of the Becuna's paperwork was processed in the yeoman's shack.

Anything involving paper — reports, commendations, personnel records, order forms for supplies — went through the yeoman.

In the control room, Williams allowed me to climb the ladder up into the conning tower, which is usually off-limits to public tours.

The conning tower is where the commanding officer usually issued orders from.

Once I successfully hoisted myself through the narrow hatch into the small space, I looked into one of the periscopes and saw the Olympia warship docked next door.

The conning tower contained the Becuna's main helm with the steering wheel and navigation equipment.

The Becuna used red lights for night operations and combat because it's easier for one's eyesight to adjust to total darkness from red light than from bright white light. This allowed crew members with lookout duties to see in the dark more quickly. It also would save precious time if the ship lost power and the control room went completely black.

Back down the ladder in the control room, an auxiliary helm served as a backup just in case the conning tower flooded or became inoperable.

The conning tower could be sealed off in an emergency.

An inclinometer showed the angle of the submarine with a floating bubble, similar to a carpenter's level.

In a nod to the inclinometer, submarine crew members were nicknamed "bubbleheads."

The hydraulic manifold was nicknamed the "Christmas tree" because of its red and green lights indicating open and closed hatches.

Crew members opened and closed the hatches with levers that used hydraulic pressure.

"Each set of these lights corresponds to a hatch or a hull opening somewhere on the boat," Williams said. "If the light is green, that hatch is closed. If it is red, that hatch is open. When you are diving, you want what's called a green board."

The compressed air manifold distributed compressed air throughout the submarine.

Compressed air had many crucial jobs on a submarine. It started the engines, fired torpedoes, flushed toilets, and blew water out of the ballast tanks to allow the ship to surface.

The radio room was used for external communications and covert listening.

"This is one of those spots that still needs some research and trying to figure out what was where because, obviously, a lot of that listening stuff was taken out because it is sensitive material," Williams said.

Chefs on board the Becuna prepared the crew's meals in the main galley.

Crew members ate four meals a day: breakfast, lunch, dinner, and midnight rations also known as "mid-rats," which were usually dinner leftovers or a simple meal like cold-cut sandwiches.

The crew ate meals, watched movies, and played games in the crew's mess.

The game boards on the tables are original to the submarine.

A hatch in the floor of the crew's mess led down into the refrigerator where fresh food was stored.

Perishables usually lasted through the first two or three weeks of a two-month-long patrol. After that, the crew subsisted on canned goods.

I climbed down through another hatch in the floor to see the sonar equipment — a bonus that's not usually included in the public tour.

Sonar stands for "sound navigation and ranging."

The Becuna used both active and passive sonar to detect other underwater vessels. Active sonar sends out pings of sound that bounce off other objects, revealing their location and distance. Passive sonar simply listens for activity.

The Becuna mostly used passive sonar to remain undetected.

The sonar room below the crew's mess was soundproofed to block out the noise of movie screenings and chatter from above.

The crew's berthing contained 35 bunks.

The forward and aft torpedo rooms each contained 15 bunks. Together with the 35 beds in the crew's berthing, there were 65 bunks for the 67 non-officer crew members on board.

"Hot racking," or the rotating use of bunks by crew members with different shifts, didn't need to happen very often.

The Becuna featured two engine rooms, each containing two General Motors V16 diesel engines.

The engines, which ran at 1,600 horsepower each, may have been nicknamed "Huff," "Puff," "Grunt," and "Groan." These names are painted on the engines, but the museum isn't sure if they're historically accurate.

In the forward engine room, evaporator distillers filtered ocean water to maintain the submarine's crucial supply of fresh water.

The distillers could produce up to 1,000 gallons of fresh water per day, but actually produced around 600 gallons in practice because they often burned out and required maintenance, Williams said.

Tanks on either side of the submarine could hold 4,000 gallons of fresh water. The Becuna burned through the supply quickly at a pace of about 5,000 gallons a week.

Most of the distilled water went toward keeping the submarine's batteries hydrated as they burned off their electrolyte fluid. The rest was used for cooking, drinking, and cleaning machinery. Hygiene was last on the list, with enough water left over for each crew member to take a two-minute shower once a week.

Crew members oversaw the submarine's propulsion and electricity use from the maneuvering room.

The bus switches directed electricity to its generators, which powered its motors.

The aft torpedo room featured real demilitarized torpedoes on display.

Each Mark 14 torpedo weighed 3,000 pounds.

The aft torpedo room also featured amusing recreations of original comics poking fun at crew dynamics on board.

"The takeaway here is everything old is new again," Williams said. "This is one of your first memes, essentially."

My Becuna tour provided a fascinating look into a rare, little-known aspect of submarine history.

By helping to preserve the story of the only remaining submarine with GUPPY 1-A modernization, Williams hopes to show visitors that submarine history encompasses more than just World War II and nuclear vessels.

"Very few people understand or know that there is a period of about 30 years between the end of the war and when the nuclear fleet really came online that these diesel boats are doing the things that we tend to associate with the nuclear," he said. "That is the story we're trying to tell."