How Do Submarines Work?

Submarines are one of the most remarkable machines humans have ever built. They spend their lives in an environment that would kill us in minutes, yet the people inside eat breakfast, sleep, and work as if they were on the surface. So how does that actually work?

The Basic Idea: Buoyancy

Everything that floats in water follows a simple rule. If an object is lighter than the water it pushes aside, it floats. If it’s heavier, it sinks. A submarine exploits this rule by constantly adjusting its own weight.

When a submarine wants to go down, it takes on water. When it wants to come up, it pushes that water out. The mechanism is straightforward, but the engineering to make it happen reliably at depth is anything but.

Ballast Tanks: The Key to Going Up and Down

A submarine has tanks called ballast tanks running along its hull. When the sub is on the surface, these tanks are mostly full of air. That air is light, so the sub floats.

To dive, valves at the top of the tanks open. Seawater rushes in from below, displacing the air. The sub gets heavier. As the weight of the sub approaches the weight of the water it displaces, the sub begins to sink. This is called “negative buoyancy.”

To surface, the sub uses high-pressure air stored in tanks on board. It blows that air into the ballast tanks, forcing the seawater back out. The sub gets lighter and rises. This process is called “blowing the ballast.”

Modern submarines can also hover at any depth without ascending or descending. They do this by making tiny adjustments to the amount of water in the tanks, keeping the sub perfectly balanced against the surrounding pressure. This is called “neutral buoyancy,” and it is what allows submarines to drift silently without surfacing.

Pressure: The Ocean’s Constant Threat

The deeper you go underwater, the greater the pressure. At sea level, the air around us presses down with about 14.7 pounds per square inch. Go down 10 meters and the water above you adds another 14.7 pounds of pressure per square inch. By 100 meters, the pressure is roughly 10 times what we feel at the surface.

A submarine’s hull has to resist this pressure from all sides. If a seal fails or a weld cracks, water rushes in and the results are catastrophic. Naval architects design submarine hulls as nearly perfect cylinders or spheres because these shapes distribute pressure evenly, preventing weak points.

According to the U.S. Navy’s official fact files, modern submarines are built with hulls of high-strength steel engineered to operate at significant depths, with safety margins built into every design. Going beyond a hull’s rated depth risks the structure buckling under the strain.

Staying Alive Inside

A submarine underwater is a closed system. The crew breathes the same air over and over. This is managed in two parts: supplying oxygen and removing carbon dioxide.

Nuclear submarines solve the oxygen problem elegantly. They electrolyze seawater, splitting it into hydrogen and oxygen. The oxygen is released into the atmosphere of the sub. Diesel-electric submarines, which are more common globally, carry compressed oxygen or use devices called oxygen generators that chemically produce oxygen. They also surface briefly or use a snorkel mast to draw fresh air when near the surface.

Carbon dioxide is scrubbed from the air using chemical systems. Lithium hydroxide canisters absorb CO2 as the crew breathes. These systems are reliable but have finite capacity, which is why very long deployments require either large stores of scrubber chemicals or nuclear power to keep producing fresh oxygen.

The temperature inside a submarine also matters. Ocean water is cold, often near freezing at depth. Without active heating and cooling systems, the interior would swing wildly with the external temperature. Submarines run heating and air conditioning constantly, keeping the crew comfortable and preventing condensation that could damage electronics.

Propulsion: How They Move

Nuclear reactors power most modern military submarines. A nuclear reactor heats water into steam, which drives turbines that spin a propeller or pump jet. The reactor uses nuclear fission, the same process that powers nuclear power plants on land. Because nuclear fuel lasts for years, a nuclear submarine can operate for months without refueling.

Diesel-electric submarines are more common outside of major navies. They run diesel engines when on the surface or snorkeling, which charge large battery banks. When submerged, they switch to battery power, which is silent and efficient but runs out after days or weeks depending on speed. This gives diesel-electric subs important advantages in coastal waters and for short missions, but limits their endurance compared to nuclear boats.

A key metric for any submarine is how quietly it can run. A silent submarine is nearly impossible to detect. Navies spend enormous effort damping engine vibrations, covering machinery with sound-absorbing materials, and designing propellers that do not create cavitation, which is the noisy formation of tiny bubbles when a propeller spins too fast in water.

The United States Navy’s Virginia-class submarines are among the quietest ever built, using pump-jet propellers and advanced acoustic quieting technology. Soviet and Russian submarines historically were louder, which gave NATO forces an acoustic advantage during the Cold War.

Seeing Underwater: Sonar

Submarines cannot see far through water. Water bends and absorbs light, making optical vision nearly useless beyond a few tens of meters. Instead, submarines use sound.

Active sonar sends out a pulse of sound and listens for the echo bouncing back from objects. It can detect other vessels, the seafloor, or underwater mountains. The problem is that active sonar also announces your presence to anyone else listening nearby.

Passive sonar is the preferred method when stealth is required. The submarine simply listens, collecting all the sounds in the water. Every vessel has a unique acoustic signature, like a fingerprint. Experienced sonar operators can identify a specific submarine class, and sometimes even an individual boat, just from the sound it makes.

The U.S. Navy operates a vast network of underwater microphones called SOSUS, the Sound Surveillance System, which spans the Atlantic and Pacific oceans. Originally built during the Cold War to track Soviet submarines, SOSUS can detect and track submarines across entire ocean basins.

Navigation Without Satellites

GPS does not work underwater. Radio signals from satellites cannot penetrate more than a few meters of water. Submarines therefore rely on inertial navigation systems, or INS.

An inertial navigation system uses extremely precise gyroscopes and accelerometers to track every movement the submarine makes. From a known starting position, the INS continuously calculates where the submarine must be based on how fast it has traveled in each direction. It is like dead reckoning, but with instruments precise enough to track movement down to fractions of a degree.

INS is accurate but drifts slightly over time. Submarines periodically surface or raise a periscope to get a navigation fix or use other methods to correct this drift. Some modern systems also use bottom contour matching, comparing the ocean floor topography measured by sonar against stored maps to pinpoint location.

The Periscope: A Peek Above the Surface

The periscope is one of the most iconic submarine features, but modern submarines do not use it nearly as often as movies suggest. Raising a periscope breaks the surface, which makes the submarine visible to radar and nearby ships.

Most of the time, submarines navigate, communicate, and gather intelligence without surfacing. They use satellite communications, radar, and electronic sensors while at periscope depth, just barely under the surface. Only when stealth is less critical or when batteries need recharging do submarines spend extended time on the surface.

The periscope itself has evolved from a simple optical tube into a sophisticated electronic eye. Digital cameras and sensors mounted on a retractable mast send imagery to screens inside the control room. The officer on watch can scan the horizon without ever putting an eye to the eyepiece.

Going to the Deepest Places

Military submarines operate in the hundreds of meters range, but some research submersibles go far deeper. The Trieste, a bathyscaphe built in the 1950s, reached the Challenger Deep in the Mariana Trench in 1960, the deepest point in the world’s oceans, at nearly 11,000 meters. The pressure at that depth is over 1,000 times atmospheric pressure at the surface.

More recently, filmmaker James Cameron built the Deepsea Challenger, a solo submersible that reached the same point in 2012. The Woods Hole Oceanographic Institution’s Alvin submersible has been operating since 1964 and has been upgraded multiple times to reach deeper depths, serving as one of the most productive deep-sea research vessels in history.

A Remarkable Piece of Engineering

Submarines work by combining physics principles that are centuries old with engineering precision that pushes the limits of what materials and manufacturing can achieve. The ballast system uses the same buoyancy principle that Archimedes described over 2,000 years ago. The hulls withstand forces that would crush an unaugmented human body instantly.

Modern submarines can stay submerged for months, cross oceans underwater, carry weapons that can reach targets thousands of kilometers away, and move through the deep ocean in near-silence. They are simultaneously one of the oldest vehicle designs and one of the most technologically advanced.

The next time you hear about a submarine crossing an ocean completely undetected, remember that it is the result of thousands of people managing hundreds of complex systems, all working together in an environment humans were never built for. That is what makes submarines so remarkable: they take the rules of physics that we learned from a world above the waves and apply them to a world most of us will never see.