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How Deep Can We Go?

Text by Dennis Guichard
Understanding the human limits of deep-water saturation diving.

I must be honest and clarify upfront that I’m a total shallow-water diving whimp. I’m not against taking risks if those are adequately calculated and prepared for but I do feel much more comfortable frolicking in the shallows than I do in the depths. I just know that if I have any equipment malfunction that I’m in relatively safe reach of the surface, and the corals & fish life are frequently so much more abundant on the shallower reefs.

I do however have the utmost respect and admiration for the people doing deep water dives and there is unavoidably a part of me, as I’m sure is equally true for many, that is attracted to the deep water perhaps in just conquering all the technical challenges of getting there and back safely.

When most of us think about shallow versus deep diving, our fathoming thereof is limited in conversation to water perhaps shallower than 18m depth, in comparison to the deep-water technical diving that some people are frequently doing closer to 100m depth. But then there are recreational divers also pushing the utopia of that depth concept to the extreme with open-water scuba and closed-circuit rebreathers to over 300m depth. And then again there are commercial saturation divers who are pushing the very boundaries of what seems possible with human limitations currently as deep as 701m depth.

As far back as 1959, diver Hannes Keller became interested in extensive deep diving and started developing tables for mixed gas decompression. Not a diver himself, the now-famed physician Professor Albert Bühlmann became intrigued with the project and got involved helping develop the algorithms and gas mixtures to support that work.
Keller successfully tested the new Bühlmann gas mixtures and decompression tables in Lake Zürich where he reached a record water depth of 120m, and then again in Lake Maggiore in Brissago, Switzerland, where he reached a water depth of 220m (1961) with diver Damian McLeish.

Professor Bühlmann was central to much of the early work that made deep water diving possible and many new depth records were established over time by numerous divers using his decompression algorithms: In December 1960 a depth of 250m was achieved in the French Navy chamber in Toulon. In December 1962 a depth of 305m was achieved by Hannes Keller in California with the US Navy. In October 1966 a depth of 220m was achieved off Italy with a Capshell saturation expedition. And in February 1969 a depth of 350m was achieved off the coast of England near Alverstone.

In 1975 a revolutionary new 1,000m capability hyperbaric chamber was opened at the University of Zürich funded personally by Professor Bühlmann, also with the support of Canton of Zürich, the Swiss Confederation, the French Military, and Shell International. In February 1981, dive No 373 reached 575m depth pressure in the Zürich chamber. It was a revolutionary achievement for that early time.

But deep diving isn’t without its risk and challenge - divers face a host of debilitating physiological and psychological challenges when their bodies are subjected to the intense pressures at those limiting depths.

The deep dive record for open-circuit scuba currently stands at a staggering 332.35m depth, set by Ahmed Gabr in the Red Sea in September 2014. The descent to that depth took 15-minutes whilst the ascent took some 13-hours and 35-minutes with all his decompression obligations.
Prior to that the depth record had been held by Nuno Gomes with a dive to 318.25m, also in the Red Sea in 2005, although Nuno still holds the record for the deepest cave dive at 282.6m depth (a 339m altitude-adjusted sea level equivalent depth), set in 1996 at Boesmansgat in South Africa. The woman’s deep diving record was also recently broken in Boesmansgat on 27th October 2022 by Karen van den Oever with a dive to 246.65m depth, breaking her own previous record dive to 236.04m depth.

According to Wikipedia there are only 21 recreational/technical divers who have ever descended below a depth of 240m on either open or closed-circuit scuba. The holy grail of open water diving below the 300m limit has reportedly only been achieved six times since John Bennett first broke that barrier in 2001. The deepest dive record of 316m, on a closed-circuit rebreather, was set by Jarek Macedoński in Italy, on 10th October 2018.

Defying all the odds of nitrogen narcosis and central nervous system oxygen toxicity, Dan Manion amazingly set the current record for a deep dive on air at 155m depth. Manion reported he was almost completely incapacitated by nitrogen narcosis and has no recollection of his time at that depth.
The list of names of divers who have attempted setting various deep-water records, but died in the process, is sadly an extensive one. The deep is not for the feint hearted.

The realms of ‘properly deep’ diving however belong to the superheroes of commercial diving - the saturation divers. They who can master the deep likewise master a substantial commercial and financial advantage over their competitors and so the race to develop technologies and decompression algorithms bespoke to those challenges has been a fiercely contested one. There are seemingly few organisations who have the capability to do saturation dives to even 300m, so the realms of achieving dive depths deeper than that to perhaps 500m, or up to a 1,000m, are even fewer again.
Technology is perhaps the ‘simple’ part, as we already have that available to permit us access to the deepest oceans, with Victor Vescovo achieving a manned-vessel depth of 10,927m at the base of the Mariana Trench in April 2019. Putting humans into the water is however quite another challenge altogether limited it seems by the body’s capability to cope with the immense hydrostatic pressures.

In regular scuba diving we are limited by progressive risk of incapacitating nitrogen narcosis at anything over perhaps 18m depth and limited beyond 30m on air because the exertion of breathing enhanced gas density beyond 5.2g/litre at that depth can trigger an exponential risk of carbon dioxide build-up in the body. Carbon dioxide can also have a narcotic impact on our ability to think clearly as it has a narcotic potential 20-times more than nitrogen, causing increased blood flow to the brain and heightening our risk of severe neurological decompression sickness.
n dives below 50m depth, a reduction in endothelial function of the pulmonary arteries commonly occurs. Interestingly the mechanisms were shown to be different between males and females with the dysfunctionality occurring more severely in males from a change in nitric oxide bioavailability, whereas in females there was a resultant imbalance of circulating prostanoid signalling hormones which function to control vasoconstriction and are mediators of inflammatory reactions.

In dives to 60m performed by the Poland Naval Academy in 2021, elevated biomarkers of endothelial dysfunction (ICAM-1 and VCAM-1) were identified. These help regulate inflammation-associated vascular adhesion and the migration of white blood cells to repair endothelial damage.
The endothelium is sensitive to oxidative stress and these biomarkers might be a key measure as a decompression sickness stress marker.
66m depth is of course the maximum recommended depth for dives on air because of the high risk of central nervous system oxygen toxicity convulsions, let alone the severity of nitrogen narcosis at this depth.

In deep technical and commercial diving beyond 50m depth, it is common to dilute the nitrogen component of our breathing gas with a less narcotic inert gas like helium. This permits us to dive deeper with a clearer mind but from around 150m depth helium itself becomes problematic in causing high pressure nervous syndrome (HPNS). HPNS manifests with hyperexcitability, tremors, changes in brain wave electrical activity, alterations in cognitive function, and even convulsions at more extreme depths.
Strategies to minimise HPNS include slowing the descent rate on compression, including stage stops to permit diver acclimatisation, limiting excessive oxygen partial pressure, and diluting the helium with other inert gasses like nitrogen or hydrogen, although those equally each have narcotic potency of their own. Diver selection is also key for deep water saturation operations as individual susceptibilities are common and dive teams are often carefully selected by saturation supervisors to suit the dive operation intended.  

Incorporating these dive management adjustments has permitted divers to achieve depths of up to 534.4m (Comex Hydra 8 excursion in 1988) in the Mediterranean Sea, as well as a record simulated depth pressure of 701m in a hyperbaric chamber. However, despite these strategies using various gas mixtures, many exploratory dives have had to be aborted because divers have often experienced psychotic disorders like hallucinations and delirium, which seem to be as a result of the combined various narcotic potencies of the mixed gas solutions. As you solve one problem you invariably create another it seems.
In a saturation dive to 273m in July 2017, the deepest performed in Australian waters, many of the dive team reported suffering hallucinations, nausea, tremors, and cognitive impairment, allegedly due to too rapid descent procedures, that seems to have left many with residual injury and which has resulted in legal proceedings against the dive operator by the Commonwealth Director of Public Prosecutions in Australia.  

In dives beyond about 260m it has been seen that divers can also experience ‘nocturia’ where they have sustained diuresis needing to urinate endlessly and thus suffer from disturbed sleep that quickly leads to exhaustion. With dives from about 450m depth, disruption in EEG brain wave activity has been seen as well as disruption of REM sleep and awake cycle periods ultimately leading to diver exhaustion.

In deep saturation dives at 500m depth, a decrease in small airway and diffusion function was shown through a Chinese study, with divers suffering decreased Forced Vital Capacity and Forced Expiratory Volume for up to 3-days post dive, thought to be induced through airway lesions.

On 20th November 1992, Greek diver Theodoros Mavrostomos achieved a chamber dive depth of 701m with the French COMEX Hydra 10 operation, breathing a mixture of Hydrogen/Helium/Oxygen (Hydreliox), after the rest of this team were held incapacitated at 675m depth. The slow controlled descent for that dive took 13-days, the team spent 3-days at 675m depth pressure with a short 2-hour excursion to 701m by Theodoros, and then took just over 23-days of decompression time to get back safely to the surface. A remarkable achievement that hasn’t been matched or surpassed now for 30-years.

Because of the limiting constraints of psychotic-like disorders due to inert gas narcosis at great depth, regardless of the gas mixtures used, it is estimated that the maximum operating depth limit for humans might be in the region of 1,000m depth. Even with the administration of inhalational anaesthetics it is estimated that no human dives would be possible beyond 1,200m depth even in divers showing a low sensitivity to Hydreliox narcosis.


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