The human body is resilient, but it is quickly overwhelmed by the crushing environment of the deep ocean. How much pressure a human can withstand depends entirely on whether the body is unprotected or encased in technology. Hydrostatic pressure, the force exerted by the weight of the water column above, increases relentlessly with depth. This force is measured in atmospheres absolute (ATA), where every 10 meters of seawater adds one additional atmosphere of pressure. While the body’s fluid-filled tissues are incompressible, the air-filled spaces are susceptible to this extreme force, setting the initial limits for human survival.
How Hydrostatic Pressure Affects the Human Body
Boyle’s Law states that the volume of a gas is inversely proportional to the pressure exerted on it. As a diver descends, the increasing external hydrostatic pressure compresses the air within the body’s cavities, including the lungs, sinuses, and middle ear. At just 10 meters, the pressure doubles to 2 ATA, halving the volume of any trapped gas.
The reduction in gas volume creates a pressure differential between the air spaces and the surrounding tissues. Failure to equalize this pressure results in a condition known as barotrauma, or “squeeze.” Middle ear barotrauma is the most common injury, occurring when the diver cannot add air to the inner ear cavity to match the external pressure.
Pulmonary barotrauma, where the lung tissue is damaged by compression, occurs at greater depths. For a breath-hold diver, the lungs reach their residual volume—the minimum volume they can be compressed to—at a depth of 30 meters (4 ATA). Beyond this depth, the mechanical limit of the lungs becomes a significant barrier unless a physiological adaptation occurs.
The Mechanical Limits of Unprotected Diving
The mechanical limit of the unprotected human body is deeper than once thought, thanks to the mammalian diving reflex. This reflex is triggered by cold water contact and breath-holding, initiating protective measures. The heart rate slows dramatically (bradycardia), and blood vessels in the extremities constrict (peripheral vasoconstriction), shunting oxygenated blood toward the body’s core organs.
The “blood shift” is the most profound adaptation, protecting the lungs from compression. As the lungs are squeezed past their residual volume, blood plasma and red blood cells are drawn from the peripheral circulation into the thoracic cavity and the capillaries surrounding the alveoli. This fluid effectively replaces the compressed air volume, preventing the lungs from collapsing entirely and delaying the point of physical failure.
Elite free divers demonstrate the limits of this adaptation, pushing the body to its mechanical extreme on a single breath. The deepest recorded “No-Limits” free dive, using a weighted sled for descent, reached 253 meters (26 ATA), although this resulted in severe decompression sickness. A more relevant measure of physical tolerance is the Constant Weight No Fins discipline, where the diver swims down and up without assistance, with the current record standing at 103 meters (11.3 ATA).
Gas Toxicity and Neurological Barriers
While breathing compressed gas, divers encounter limits imposed by gas toxicity long before reaching crush depth. This is because the partial pressure of the gases breathed increases proportionally with the ambient pressure, causing inert and even life-sustaining gases to become toxic. For a diver breathing standard air, the first functional barrier is nitrogen narcosis, which manifests as a reversible impairment of cognitive and motor skills.
Nitrogen narcosis typically becomes noticeable at depths around 30 to 40 meters (4 to 5 ATA) and poses an unacceptable safety risk past 60 meters. A second limit is Central Nervous System (CNS) Oxygen Toxicity. Oxygen becomes a neurotoxin when its partial pressure exceeds 1.4 ATA, a level reached at 66 meters (7.6 ATA) when breathing air. Exposure to oxygen above this threshold risks an immediate and often fatal convulsion.
To dive deeper, saturation divers use specialized gas mixtures, such as Trimix, which replaces nitrogen and some oxygen with helium. Even helium introduces its own limit: High-Pressure Nervous Syndrome (HPNS). This neurological disorder, characterized by tremors, dizziness, and motor impairment, appears when divers pass 150 meters and is caused by the effect of extreme pressure on nerve cell membranes. The deepest saturation dive ever performed occurred in a chamber simulating a depth of 701 meters (71.1 ATA), demonstrating the chemical-functional limit of the human body under pressure while breathing an optimized gas mix.
Exceeding Natural Limits with Submersible Technology
When the human body is protected by technology, the limit of pressure tolerance shifts entirely from human physiology to the strength of engineered materials. The pressure a person can withstand is then dictated by the yield strength of the vessel they inhabit. Atmospheric Diving Suits (ADS) and deep-sea submersibles are designed to maintain an internal pressure of 1 ATA, regardless of the external hydrostatic force.
The Challenger Deep in the Mariana Trench serves as the benchmark for technological pressure tolerance. The depth there is nearly 11,000 meters, where the external pressure exceeds 1,000 ATA. Humans have successfully descended to this depth inside thick-walled bathyscaphes and submersibles, safely withstanding a pressure that would instantly compress an unprotected body to a fraction of its size.