Pressure is defined as the amount of force exerted over a specific unit of area. In the underwater environment, this force originates from the mass of the fluid pressing down. The core principle is that pressure increases in a direct, linear relationship with the depth submerged. Descending twice as deep results in double the water pressure. The physical laws dictating this phenomenon are constant, creating a highly predictable pressure environment.
The Weight of the Water Column
The underlying physics of underwater pressure, known as hydrostatic pressure, is a direct consequence of gravity acting upon the water’s mass. Gravity pulls the water mass downward, creating a force that accumulates as the water stacks up. A submerged body experiences the cumulative weight of all the water situated directly above it.
The pressure felt at any depth is determined by the height of the water column, the fluid’s density, and the acceleration due to gravity. Water is significantly denser than air, which causes pressure to increase rapidly with descent. The scientific formula for calculating hydrostatic pressure is \(\rho g h\), where \(\rho\) (rho) is the fluid density, \(g\) is the acceleration due to gravity, and \(h\) is the depth of the column.
Density is an important factor; saltwater is slightly denser than freshwater due to dissolved salts, leading to a marginally higher pressure increase at the same depth. Gravity ensures that the force exerted by the water column is transmitted equally in all directions at a specific depth. This means an object submerged at 33 feet (about 10 meters) experiences the same force on its sides, top, and bottom.
The continuous transmission of this force is why deep-sea submersibles are often designed with spherical shapes, which are best suited to withstand omnidirectional compression. The rate of pressure increase is consistent throughout the oceans. For every 33 feet (approximately 10 meters) of descent in saltwater, the pressure increases by one atmosphere (ATM), or approximately 14.7 pounds per square inch (psi). This predictable accumulation of force dictates the environment of the deep sea.
Understanding Absolute Pressure
The pressure experienced underwater does not begin at zero at the surface. Before entering the water, the body is already under the influence of the Earth’s atmosphere, which exerts its own weight. This initial force, known as atmospheric pressure, is the weight of the air column extending from the surface.
Atmospheric pressure is standardized as 1 ATM (atmosphere) at sea level, equating to about 14.7 psi. The total force experienced by an object underwater is the combination of water pressure and this initial atmospheric pressure. This combined measurement is referred to as absolute pressure.
To determine the total pressure at a given depth, the 1 ATM of surface pressure must be added to the pressure exerted solely by the water. The pressure generated by the water column alone is called gauge pressure. Gauge pressure is the value typically read on a diving depth gauge, which is calibrated to ignore the 1 ATM of surface pressure.
For example, at a depth of 33 feet (10 meters), the water column contributes 1 ATM of pressure. The absolute pressure is calculated by adding the surface pressure, resulting in 2 ATM total. This means the force exerted on a submerged object at 33 feet is double the force felt on the surface.
Descending to 66 feet (20 meters) adds another atmosphere of water pressure, making the total absolute pressure 3 ATM. This additive relationship demonstrates why pressure increases quickly, rising by 1 ATM for every 33 feet of descent.
How Pressure Compresses Air and Objects
The most dramatic effect of increasing underwater pressure is its interaction with gases, particularly air. While liquids and solids are generally considered incompressible, gases are highly compressible because their molecules are widely spaced. As pressure mounts, the available space for gas molecules shrinks, forcing them closer together and decreasing the volume.
This relationship, where the volume of a gas is inversely proportional to its pressure, affects anything containing air underwater. When the absolute pressure doubles from 1 ATM at the surface to 2 ATM at 33 feet, the volume of trapped air is halved. Continuing the descent to 99 feet (4 ATM) reduces the original volume of air to one-quarter of its size.
This compression affects flexible containers and air spaces within the human body, such as the lungs, middle ear, and sinuses. The air volume within the lungs is compressed according to the depth, dramatically increasing the density of the air being breathed.
A phenomenon known as “squeeze” occurs when the external pressure becomes greater than the pressure inside an air-filled space. If the pressure in the middle ear is not manually equalized to match the surrounding water pressure, the resulting differential can cause pain and damage. Equalization involves introducing higher-pressure air from the lungs into the air space to balance the external force.
Objects like flexible plastic bottles will visibly collapse as the gas inside is squeezed into a smaller space. Rigid-walled containers, such as submarine hulls, resist the compressive force. The design of deep-diving vessels must account for the tremendous and uniform force that attempts to crush the hull.
The ability of air to compress also changes how gases interact with the body’s tissues. As the volume of breathing gas shrinks at depth, the partial pressure of each component gas, such as nitrogen and oxygen, increases significantly. This rise in partial pressure is the underlying cause of physiological effects experienced by divers, including nitrogen narcosis, an altered mental state resulting from the high concentration of nitrogen dissolved in the brain.