The ocean’s depths present an environment of crushing pressure, near-total darkness, and frigid temperatures. Venturing into this extreme realm requires overcoming profound physiological challenges to the human body. Understanding these conditions helps to clarify the limits of human presence in the deep sea.
Physiological Effects of Ocean Depth
As a diver descends, the surrounding water pressure steadily increases, impacting the body’s gases and tissues. This change is governed by gas laws such as Boyle’s Law, which states that the volume of a gas decreases proportionally as pressure increases. This compression also affects gas solubility in the body’s tissues, as described by Henry’s Law, where more gas dissolves into liquids at higher pressures.
One significant concern is decompression sickness (DCS), often called “the bends,” which occurs when dissolved inert gases, primarily nitrogen, form bubbles in tissues and blood during ascent. If a diver ascends too quickly, these bubbles expand and can cause pain, tissue damage, and even block blood vessels. Nitrogen narcosis, known as “rapture of the deep,” poses another risk, as high partial pressures of nitrogen can impair judgment, memory, and motor skills, similar to alcohol intoxication.
Oxygen, while essential for life, becomes toxic at elevated partial pressures, a condition called oxygen toxicity. Central nervous system (CNS) oxygen toxicity can lead to severe symptoms such as convulsions, vision changes, and nausea, which can be fatal underwater. At very great depths, typically beyond 150 meters, a distinct neurological disorder called High Pressure Nervous Syndrome (HPNS) can manifest. This condition, linked to helium-based breathing mixtures, can cause tremors, dizziness, nausea, and cognitive impairment, posing a limiting factor for extremely deep dives.
Human Diving with Breathing Gas Mixtures
Humans directly enter the ocean’s high-pressure environment by breathing compressed gas mixtures, primarily through scuba diving. Recreational scuba diving typically limits divers to a maximum depth of 40 meters (130 feet) to manage gas absorption and reduce risks. At these depths, divers breathe air, but the increasing pressure intensifies the effects of nitrogen narcosis and dissolved nitrogen.
To extend depth and time limits, technical divers utilize specialized gas mixtures. Nitrox, which contains a higher percentage of oxygen and less nitrogen than air, reduces nitrogen uptake and extends bottom times in shallower deep dives, but it increases the risk of oxygen toxicity at greater depths. For deeper dives, divers employ Trimix, a blend of oxygen, nitrogen, and helium, or Heliox, a mixture of oxygen and helium. Helium is used because it is an inert gas that causes less narcosis and diffuses more rapidly than nitrogen, mitigating the risks of both nitrogen narcosis and decompression sickness.
The deepest open-circuit scuba dive record was achieved by Ahmed Gabr in 2014, reaching 332.35 meters (1,090 feet) in the Red Sea. For even longer durations at depth, saturation diving is employed, where divers live in pressurized habitats, breathing specialized gas mixtures for days or weeks. This technique allows them to work at depths without daily decompression, significantly increasing their time on the seafloor. The deepest saturation dive in open water reached 534 meters (1,752 feet) by a COMEX team in 1988, using a hydrogen, helium, and oxygen mixture called hydreliox. In a hyperbaric chamber, a simulated dive to 701 meters (2,300 feet) was achieved in 1992, showcasing the physiological limits under controlled conditions.
Deep-Sea Vehicular Exploration
For exploring the ocean’s most profound depths, direct human exposure to ambient pressure is not feasible. Instead, humans rely on specialized deep-sea vehicles that maintain a surface-like, oxygen-rich environment for their occupants. These submersibles, such as bathyscaphes and modern deep-sea submersibles, are designed as robust pressure vessels capable of withstanding external pressure.
These vehicles protect their occupants by entirely isolating them from the surrounding high-pressure water. Inside, the air pressure remains near normal atmospheric levels, and oxygen is supplied, similar to breathing on land. This technological solution bypasses the physiological limitations faced by divers directly exposed to ambient pressure, allowing exploration of depths where the human body could not otherwise survive.
The deepest points in the ocean, like the Challenger Deep in the Mariana Trench, are explored using these advanced submersibles. In 1960, the bathyscaphe Trieste, carrying Jacques Piccard and Don Walsh, descended to 10,915 meters (35,810 feet) in the Challenger Deep. More recently, in 2012, filmmaker James Cameron piloted the Deepsea Challenger to a similar depth, and in 2019, Victor Vescovo reached 10,928 meters (35,853 feet) in the DSV Limiting Factor. These remarkable depths are vastly greater than those achievable by any form of direct human diving, demonstrating that while humans are present “with oxygen” in these vehicles, they are fully shielded from the extreme pressure of the deep ocean.