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. The US Navy operates some of the most advanced submarines in the world. So how does that actually work?

The short answer

A submarine goes up and down by changing its weight. It takes in seawater to sink and blows it out with compressed air to rise. Inside, the sub must produce its own oxygen and scrub out carbon dioxide because the crew breathes the same air repeatedly. A thick steel hull keeps the water out, while sonar replaces vision underwater. These simple ideas combine into a machine that can cross oceans undetected.

The full picture

Going up and down: Buoyancy and ballast

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

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.

The pressure problem

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 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 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.

Moving through water

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. The World Nuclear Association documents how naval nuclear propulsion has evolved since the 1950s.

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 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 and hearing underwater

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 has evolved

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.

Why it matters

Submarines represent a unique military capability that no other vehicle provides. They can appear without warning, launch attacks from anywhere under the ocean, and remain hidden for months. This makes them one of the most strategic assets in any navy.

But submarines also matter for science. They let researchers reach depths no diver can survive. The Trieste reached the Challenger Deep in 1960, nearly 11,000 meters down, where the pressure exceeds 1,000 times atmospheric pressure at the surface. More recently, filmmaker James Cameron built the Deepsea Challenger to reach the same point in 2012. The Woods Hole Oceanographic Institution 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.

Understanding how submarines work also reveals something about engineering itself. The same physics principles that Archimedes described over 2,000 years ago still govern buoyancy. What has changed is our ability to manufacture materials that withstand crushing pressure, build reactors that run for years without refueling, and design systems that let humans live underwater for months at a time.

Common misconceptions

Myth: Submarines only dive when their hull fills with water. Reality: Submarines do take on water, but it goes into specific ballast tanks, not the hull itself. The crew compartment stays sealed and dry. The water in ballast tanks is precisely controlled to adjust buoyancy, and the sub can blow it out at any time using compressed air.

Myth: Submarines can stay underwater indefinitely. Reality: Even nuclear submarines, which do not need to surface for oxygen, eventually need to resupply food, spare parts, and maintenance materials. Most military submarines can stay submerged for 70 to 90 days beforeCrew morale and operational tempo become limiting factors. Diesel-electric submarines are even more constrained, typically lasting days to weeks submerged before needing to surface or snorkel.

Myth: Sonar lets submarines see as well as we do above water. Reality: Sonar is nothing like vision. It only tells you whether something is out there and roughly how far away. It cannot identify what an object is with the detail your eyes provide. Submarines sometimes mistake whales for other submarines, and the ocean floor can look confusing on sonar returns.

Key terms

Buoyancy: The upward force water exerts on any object. If an object weighs less than the water it displaces, it floats. If it weighs more, it sinks.

Ballast tanks: Water tanks along a submarine hull that fill with seawater to make the sub heavier, or empty to make it lighter. The primary mechanism for going up and down.

Cavitation: The formation of tiny vapor bubbles when a propeller spins fast enough to create low-pressure regions in the water. These bubbles collapse noisily, making the submarine easier to detect.

Inertial navigation system (INS): A navigation method using precision gyroscopes and accelerometers to track a submarines movements from a known position, allowing navigation underwater where GPS does not work.

Neutral buoyancy: The state when a submarine weighs exactly the same as the water it displaces, allowing it to hover at any depth without ascending or descending.

Sonar: A system using sound waves to detect objects underwater. Active sonar sends pulses and listens for echoes. Passive sonar only listens.