For most of the last century the physiology said this was impossible.
The argument was simple and it looked sound. Lungs hold a fixed volume of air, water pressure compresses that air, and at some depth it compresses to nothing.
Below that point the chest has nowhere left to go.
Work the arithmetic and the answer came out around 100 feet for an ordinary person, with the ribcage collapsing somewhere beyond it.
Divers then went twice that deep, and four times, and eventually seven.
Which means the arithmetic was right and the model was wrong.
The chest fills with blood instead of air
What the old calculation left out is that a body is not a rigid box with a gas pocket in it.
The moment a person is immersed, blood begins moving out of the limbs and into the chest, and the deeper the dive the more of it shifts.
Something in the range of 2 to 3 pints of blood ends up in the pulmonary circulation, filling the space the compressed air has vacated.
That changes the sum completely. If the air in the lungs can only squeeze down to a liter and a half, the ceiling is shallow. If blood takes up a liter of that space, the air can squeeze to half a liter instead.
The same ribcage can then go roughly three times deeper without anything breaking.
The lungs are not resisting the pressure. They are being backfilled.
A sled, a line and a lift bag
The category this record belongs to is called No Limits, and the name is honest about what it permits.
The diver holds a weighted sled that runs down a fixed line and is dropped, which removes the work of swimming down and makes the descent far faster than a person could manage.
At the target depth the diver releases the sled and opens a gas filled bag, and buoyancy does the entire return.
No fins, no swimming, no effort at all in either direction, which is why the depths in this category are far beyond the others.
It is also the reason the discipline is regarded as the most dangerous form of the sport.
Speed is the point, and speed is also the hazard.
Twenty two atmospheres off Spetses
The dive was made on June 14 of 2007, off the island of Spetses in Greece, by an Austrian who holds 33 world records across eight disciplines.
The certified depth is 702 feet and just over an inch, measured on the line and ratified afterward.
At that depth the water is pressing at about 22 atmospheres, so a chest that held roughly 12 pints of air at the surface is down to something close to half a pint.
That is the figure worth sitting with. Roughly one part in 22 of the volume it started with, and the chest wall intact.
He surfaced without injury, and the record has stood ever since.
Five years later he went deeper still, to 831 feet, and that one went wrong.
What the record does not mention
Ten minutes after surfacing from the deeper dive he had serious decompression sickness.
He had fallen briefly asleep from nitrogen narcosis during the last part of the ascent, was flown to a hyperbaric chamber in Athens, and suffered multiple strokes from the sickness.
The early prognosis was home care and help with walking. Years of rehabilitation got most of it back, and balance and coordination problems on land remain.
Freedivers were long assumed to be immune to decompression sickness because a single breath carries so little nitrogen, in the way that a very long breath hold or a long silence in space gets read as a safe extreme.
Modeling puts the risk as negligible to about 330 feet and rising sharply after that, and one survey found 7 in 100 breath hold divers reporting stroke like events, which is not a rare outcome.
The parts of this that are borrowed
Two other tricks are doing quiet work here and both come from elsewhere in biology.
The first is the spleen, which contracts during a dive and pushes stored red cells into circulation. Elite divers have larger spleens than matched controls and empty them harder, though the measured gain in oxygen carrying is modest and the studies disagree.
The second happens before the dive begins. Divers use the mouth and throat as a pump, forcing extra air into already full lungs against a closed airway, which can raise total lung capacity by nearly half.
Seals do the first one far better and have no need of the second, because their chests are built to collapse on purpose rather than to be propped open.
That difference is the whole gap between an animal that lives at depth and a person visiting it, as the diver’s own account of the discipline makes clear.
The weighted sled is what makes the depth possible.
The blood shift is what makes it survivable.
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