Crush depth refers to the extreme pressure beyond which an object or organism can no longer withstand external forces, leading to structural failure. While often used in discussions about submersibles, the principle also applies to the biological limits of living organisms, including humans, when exposed to increasing underwater pressures.
The Physics of Pressure Underwater
Pressure in an aquatic environment increases significantly with depth due to the accumulating weight of the water column above. At sea level, air pressure is approximately one atmosphere (ATA), equivalent to 14.7 PSI or 1 bar. As one descends, pressure increases by about one atmosphere for every 10 meters (33 feet) of depth. This means that at 100 meters, the total pressure is around 11 ATA (1 atmosphere from air, plus 10 from water).
How Pressure Affects the Human Body
The human body is largely composed of water and incompressible tissues, which are not significantly affected by increasing pressure. However, gas-filled spaces within the body, such as the lungs, sinuses, and middle ears, are highly susceptible to pressure changes. As pressure increases during descent, the gases in these cavities compress, which can lead to discomfort or injury known as barotrauma if not properly equalized.
Beyond direct physical effects on air spaces, dissolved gases can also cause physiological changes. Nitrogen narcosis, sometimes called “rapture of the deep,” occurs when nitrogen gas, breathed at increased partial pressures, dissolves into the nervous system. This can lead to impaired judgment, euphoria, disorientation, and loss of consciousness, typically becoming noticeable at depths beyond 30 to 40 meters (100 to 130 feet). Oxygen can also become toxic at high partial pressures, leading to central nervous system symptoms like convulsions and muscle twitching, or pulmonary issues such as coughing and chest pain.
Defining “Crush Depth” for Humans
For an unprotected human, “crush depth” does not involve the entire body being compressed into a solid mass, but rather the catastrophic failure of its gas-filled structures. The primary determinant is the extreme compression and eventual collapse of the lungs and other air cavities.
As an unprotected individual descends, the lungs continue to shrink due to external pressure, reaching their residual volume. Beyond this point, the pressure differential between highly compressed gas inside the lungs and relatively incompressible blood and tissues can cause severe damage. This leads to catastrophic barotrauma, where the lungs rupture, blood vessels burst, and in extreme cases, the chest wall may cave in.
While not a precise single number due to individual physiological variations and the rate of descent, this fatal point generally occurs at pressures around 10 to 12 atmospheres absolute, corresponding to depths of 90 to 110 meters (300 to 360 feet).
Human Deep-Sea Exploration and Protection
Despite the severe limitations of the unprotected human body underwater, technology allows for extensive deep-sea exploration. Submersibles and atmospheric diving suits (ADS) are engineered to maintain an internal pressure of one atmosphere, shielding occupants from external forces. These protective systems eliminate the physiological dangers of high pressure, such as barotrauma, nitrogen narcosis, and oxygen toxicity. Modern submersibles have enabled humans to reach the deepest parts of the ocean, with the deepest manned dive recorded by Victor Vescovo to 10,928 meters (35,853 feet) in the Mariana Trench. In contrast, the deepest breath-hold (freediving) record, achieved by Herbert Nitsch, stands at 253 meters (830 feet), highlighting the vast difference between unprotected human physiology and technologically protected environments.