Standard Operating Depths
Submarines operate at a range of depths depending on their purpose and design. Military submarines, such as attack submarines (SSNs) and ballistic missile submarines (SSBNs), generally operate from a few hundred feet to over 1,000 feet (approximately 300 meters) for stealth and maneuverability.
Research submersibles and some specialized civilian vessels are designed to go deeper, exploring ocean trenches and conducting scientific studies. Some research submersibles routinely descend to several thousand feet for deep-sea observations. Tourist submarines, intended for recreational viewing, operate at much shallower depths, typically limited to a few hundred feet for safety and clear visibility.
A key distinction exists between a submarine’s “operating depth” and its “test depth.” Operating depth refers to the shallower depths where a submarine typically conducts routine missions. Test depth is the maximum depth a submarine is designed to safely reach during trials, representing its structural limit.
Engineering for Extreme Pressure
Operating in the deep ocean presents a significant engineering challenge due to hydrostatic pressure. As a submarine descends, the weight of the water above it increases, exerting force on its hull. For every 33 feet (10 meters) of descent in seawater, pressure increases by approximately one atmosphere, or 14.7 pounds per square inch (psi).
To withstand these forces, submarine hulls are constructed from specialized, high-strength materials. Advanced steels, titanium alloys, and some composites are used. The shape of the hull is also critical, with cylindrical or spherical sections distributing pressure more evenly than flat surfaces.
The design accounts for the “crush depth,” a theoretical point at which the hull would structurally fail and implode. Engineers incorporate safety margins, ensuring the test depth is well within the hull’s ultimate strength.
Controlling Submarine Depth
Submarines precisely control their vertical movement by manipulating buoyancy. To descend, valves open, allowing seawater to flood large ballast tanks, increasing the submarine’s weight and making it negatively buoyant.
To ascend, compressed air is forced into the ballast tanks, expelling seawater. This decreases the submarine’s weight, causing it to rise. For maintaining a specific depth or neutral buoyancy, water in the ballast tanks is adjusted until the submarine’s weight matches the buoyant force of displaced water.
Beyond buoyancy control, submarines also use hydroplanes, horizontal fins resembling airplane wings, for dynamic depth control. When moving forward, these hydroplanes can be angled to generate lift or downforce, allowing for subtle depth changes or maintaining a steady course. This combination provides precise maneuvering capabilities.
The Deepest Dives
Specialized submersibles have ventured into the deepest parts of the ocean. The Mariana Trench in the western Pacific Ocean is the deepest known oceanic trench on Earth, reaching depths of nearly 36,000 feet (approximately 10,972 meters).
One of the earliest manned descents into the Mariana Trench was made by the bathyscaphe Trieste in 1960, reaching the Challenger Deep. In 2012, film director James Cameron successfully piloted the Deepsea Challenger to the same location. These dives required significant engineering solutions to withstand pressures exceeding 16,000 psi.
Victor Vescovo, in his submersible Limiting Factor, completed multiple dives to the Challenger Deep between 2019 and 2020, becoming the first person to repeatedly visit the location.