The maximum depth a human can reach underwater depends entirely on the technology and method used to manage hydrostatic pressure. The question of how deep a human can go underwater does not have a single answer; the limit depends entirely on the technology and method used to manage hydrostatic pressure. At sea level, we experience one atmosphere of pressure, but this pressure increases by another atmosphere for every 10 meters of descent. While the body is mostly incompressible water, the air spaces within the body present a significant physiological challenge. Maximum depth is determined by whether a person relies on inherent physical limits, uses specialized breathing gas mixtures, or is sealed inside a rigid protective vessel.
The Limits of Breath-Hold Diving
Without external breathing apparatus, the human body’s tolerance for depth is governed by two interconnected physiological responses. The first is the mammalian dive reflex, triggered by cold water contact and breath-holding, which significantly slows the heart rate (bradycardia). This reflex also causes peripheral vasoconstriction, where blood vessels in the extremities constrict, redirecting oxygen-rich blood toward the brain, heart, and lungs to conserve oxygen.
The second major challenge is the physical compression of the lungs due to Boyle’s Law, which states that the volume of a gas decreases proportionally as pressure increases. As a diver descends, the air in their lungs is compressed. Historically, it was theorized that a diver would suffer a catastrophic lung collapse, or “lung squeeze,” around 40 to 50 meters. To counteract this, the body employs a remarkable mechanism called blood shift, drawing incompressible blood plasma and red blood cells into the chest cavity. This fluid fills the space created by the compressed air, protecting the lungs from tissue damage.
Elite free divers have pushed far past the theoretical 50-meter limit by maximizing the blood shift response and using specialized breathing techniques. The deepest human descent on a single breath, using an assisted sled in the ‘No Limits’ category, has exceeded 214 meters. In the ‘Constant Weight’ category, where the diver swims down and back up unassisted, the record is over 130 meters. These feats demonstrate the extraordinary plasticity of human physiology under extreme hyperbaric conditions.
Extending Depth with Specialized Gear
When divers use breathing equipment, the limitation shifts from physical lung collapse to the chemical and neurological toxicity of breathing gases under high pressure. Standard compressed air becomes toxic at depth because the partial pressures of its components—nitrogen and oxygen—rise dangerously. Around 30 meters, the high partial pressure of nitrogen causes nitrogen narcosis, a reversible impairment of mental and physical function similar to alcohol intoxication.
The partial pressure of oxygen also becomes a severe threat, leading to central nervous system oxygen toxicity, which can cause convulsions and death. To mitigate these hazards, technical divers replace a portion of the nitrogen in their breathing gas with helium, creating a mixture called trimix. Helium is non-narcotic and less dense, allowing for deeper dives while reducing mental impairment and the physical effort required to breathe the gas.
High Pressure Nervous Syndrome (HPNS)
Even with optimal gas mixtures, increasing pressure introduces High Pressure Nervous Syndrome (HPNS). This syndrome is characterized by symptoms like tremors, fatigue, and muscular jerks, and it manifests at depths greater than 150 meters. HPNS is caused by the direct physical compression of nerve membranes and is the main physiological constraint for the deepest assisted dives. Divers can counteract HPNS by adding a small amount of nitrogen back into the helium-oxygen mix, leveraging nitrogen’s mild narcotic properties to stabilize the nervous system.
Depth Records
Using these advanced techniques and specialized equipment, the deepest open-water scuba dive recorded reached approximately 332 meters. The deepest simulated dive in a hyperbaric chamber has exceeded 700 meters.
Maximum Depth in Pressure Vessels
To descend to the absolute deepest parts of the ocean, physiological constraints must be bypassed by completely isolating the body from ambient pressure. This is achieved using pressure vessels that maintain a constant internal pressure of one atmosphere, regardless of external depth. Atmospheric Diving Suits (ADS) are small, articulated submarines worn by a single person. These suits allow divers to work at depths of up to 700 meters without the risk of decompression sickness or gas narcosis, since the internal pressure remains constant.
For the ultimate depth, humans rely on deep-sea submersibles, where the limit is determined solely by the engineering strength of the hull. The deepest point on Earth is the Challenger Deep in the Mariana Trench, reaching approximately 10,935 meters. Despite the crushing pressure—over 1,000 times greater than at sea level—humans have successfully piloted submersibles to the floor of the Challenger Deep. In this context, the maximum depth a human can go is the deepest point in the ocean, a limit set by materials science and mechanical design, not biology.