Next time youre hanging off a deco line without a care in the world as your dive computer calculates times for your safety stops, spare a thought for those pioneers whose bravery and genius made it all possible.
Had it not been for the work of father and son team John S Haldane (1860-1936) and John Burdon Sanderson (Jack) Haldane (1892-1964), its possible that the incident-free dive on that wreck in 40m you have just enjoyed would not have been incident-free at all.
These two Scots, working independently of one another for most of their careers, pushed the limits of science in their time as well as their own physical limits to make diving safer for everyone. It is arguable that, of all the advances in diving made over the past century, no others working in the field have made such significant contributions towards reducing the number of diving fatalities.
John S Haldane trained in medicine at Edinburgh University but became bored after practising for a very short time and went instead to work as a lowly demonstrator at University College, Dundee.

J His true passion, however, lay elsewhere. Far from the sterile surroundings of the hospital, J S Haldane sought to immerse himself in the most unhygienic situations possible. He went in search of foul smells and contagious diseases - in the slums of Dundee at the time, he didn't have to look far - because he wanted to research the effect on peoples health of the quality of the air that they breathed.
 He also spent a lot of time on the London Underground, collecting air samples in a jar that he hung out of the window. Carrying around a briefcase with London Fever Hospital inscribed boldly on it, he often found he had the carriage to himself!
He quickly proved that the level of carbon monoxide was unacceptably high, and as a result of his investigations the lines were electrified.

Later, he became fascinated by the poisonous mine gases that were the cause of frequent explosions. On hearing that another mine had exploded, he would grab his helmet and rush straight in with his gas-analysis equipment.
Knowing that his wife would fear for his safety, he used to send her telegrams. Depending on what poisonous substance he was inhaling at the time, these were often at best confused.
If he was particularly affected, he would forget having already sent one and she would then receive several from him, one after the other!

But J S Haldanes most lasting contribution came when he was approached by the Admiralty in 1905. The Royal Navy offered very little formal training for divers at the time and they were frequently pulled in from the gunnery schools, given a helmet and told to get on with it!
 Divers didnt carry their own air supply. Air was pumped into their helmets from the surface via a hose.
So long as the delivered air was at the same pressure as the surrounding water, this worked well, but if the air supply was cut off and the helmets inlet valve failed, the diver would suffer the catastrophic effect of thousands of tonnes of water collapsing in on him. This would often mean that bits of him would end up back on the surface via the tube.
The Navy appeared more concerned about the amount of work divers were able to do below 20m. If they survived, they soon complained of exhaustion, and J S Haldanes brief was to work out how to overcome this problem.

Using goats as the cheapest and most readily available subjects that approached human size, J S Haldane began his investigations in the Firth of Clyde. He found that divers exhaustion was due in part to increased carbon dioxide build-up in the helmets, or hats as they were known.
Part of the problem was the inefficient way in which air was sent down to the diver, so he designed a more effective pump. But he also observed that the levels of CO2, although higher than normal, were not high enough to cause problems if breathed in these quantities on the surface.
From this, he was able to prove that the adverse effects of breathing CO2 at depth were significantly greater.
The bends were an everyday occurrence. Divers were told to minimise this risk by ascending slowly to begin with, and then rising faster as they got nearer the surface. Thanks to J S Haldanes work, we know now that this is nonsense.
He demonstrated that decompression was most dangerous nearest the surface. As divers are now well aware, a diver ascending from 60m would have to travel 35m before the absolute presure on him was halved (7 bar to 3.5 bar), but only 15m to achieve the same result from 20m (3 bar to 1.5 bar).
So J S Haldane designed a series of tables which advocated slower ascents on approaching the surface. These tables were used by all navy divers between 1907 and 1956 with great success. Hundreds of lives were saved as a result.

J S Haldanes son Jack was a precocious child, and at the age of eight was analysing data in his fathers laboratory - when he wasnt busy inventing new and exciting ways of electrocuting his sister or otherwise incapacitating her. Like his father, he was to make a lasting impression on the history of diving and to take an extremely hands-on approach to his experiments!
Jacks first glimpse of the underwater world came at the age of 13, when he donned a hardhat and suit that was much too big for him and jumped in. Because the suit was such a poor fit, it soon filled with water at 10 metres.
Jack remained calm and managed to keep the water at chest height by experimenting with his helmet valves. He was equally unfazed when his father requested his assistance in an experimental submarine at the age of 16.
Even the Great War held few terrors for Jack. A big, aggressive man, he relished the opportunity to distinguish himself in battle.
His comrades were a little wary of him, especially in the bombing school he established. He was often seen to tamp down his pipe with a detonator while in the process of teaching his students how to avoid accidents with explosives!
After the war, Jack Haldane went to Cambridge and spent several years testing poisonous substances on himself and his friends to examine the physiological consequences.
When asked by his colleagues why he chose to use himself as a subject instead of adopting the more usual approach of experimenting on animals, he explained that an animal would not be able to describe its symptoms.
He also noted that, while there was plenty of tedious bureaucracy involved in acquiring animals on which to experiment, he could do what he wanted to himself and nobody seemed bothered.

In 1939, HMS Thetis sank in Liverpool Bay
when a torpedo tube was accidentally opened while the outer door was still open. It went down in about 50m, with its stern still above the surface, and 99 of its 103-strong crew perished.
Amazingly, the escape hatch was only 7m below the surface. The Navy wanted to know why the men on board expired so quickly, and hoped to be able to reassure their families that their deaths were painless. Jack was asked to investigate the physiological and psychological effects of being stuck in a cramped space with several other submariners, while breathing air rich in CO2 at depth and in icy waters.
His investigations were to serve as an inquiry into the disaster and also to provide a re-evaluation of the navys submarine-escape techniques.
 As always, his experiments were distinctly practical. He created an environment in the laboratory as close as possible to that of a sunken submarine, and used a tiny Siebe Gorman pressure chamber to simulate pressure at depth - a vessel which soon became known among the team as the Chamber of Horrors.
There was no light and no phone link to the outside. If submersion was necessary, the experiments would be performed in a small water-filled tank, with the air at the surface pressurised to whatever depth was required by the experiment.
Frequently the water was filled with blocks of ice to recreate Arctic waters.
Jack managed to recruit a strange mix of volunteers, including the intrepid experimental diver Martin Case (who never ate or drank during the day, but every night would venture into Soho to feast on a large meal, washed down by oceans of beer); Elizabeth Jermyn, his secretary; and the former Prime Minister of Spain! That Jack seemed to have no end of willing helpers was strange, given the nature of the experiments:
The work is of a severe character. I became unconscious. One of our subjects has burst a lungÉ six have been unconscious on one or more occasions; one has had convulsions.
On a typical day the volunteers would be gradually blown down to 100m or so (although Jacks own record for a chamber dive was 1 bar to 7 bar in one and a half minutes - the equivalent of a fighter plane in a nose dive at Mach 2!)
 Observers would study the effects of cold, increased CO2 and the extreme pressure. Exploding teeth, burst eardrums, fits and countless minor bends were all in a days work for the team.
Anyone who could prove that they had suffered a bend was given a day out of the pot to recover!
 It was said that Haldane could always be found by following the trail of blood-soaked cotton wool and tissues that he used to staunch the all-too-frequent nose-bleeds he endured.

In a related project, Jack was called on to conduct a test in a sealed chamber under water. The Navy wanted to know how long they could keep men down in submarines, and whether they would be able to leave the sub to cut through steel mesh in harbour entrances or affix bombs to berthed vessels.
After a stint in Portsmouth harbour, a crane was used to recover the chamber, but halfway through the operation an air-raid warning sounded! The crane-driver ran for shelter, leaving Jack and a colleague dangling over the water.
Having endured this, they were able to demonstrate that two men could survive for three days in a sealed environment sharing one oxygen cylinder between them to counteract the effects of carbon dioxide.
At these depths they were also able to experience first-hand and document the effects of nitrogen narcosis. Jack noted that these varied from person to person and even day by day.
However, at around 80 metres he was unable to find anyone who remained completely unaffected, although the symptoms would often seem to be very different for each individual. Feelings of faintness, intoxication and even vivid hallucinations were all reported.
Jack also tested heliox mixtures advocated by the Americans. These were purported at the time to reduce some of the effects of narcosis and to be safer to use during rapid ascents.
During these tests, Jack experienced a bend which he blamed on a bubble of helium forming in his spinal cord, and this was to give him trouble for the rest of his life.
Thereafter, he learned to be sceptical of American salesmanship, even if I learned it the hard way.
Jacks paper on these experiments, A Report on the Effects of High Pressure, Carbon Dioxide and Cold, forced the Navy to revise its submarine-escape procedures and the way in which its clearance divers operated.
As a result of his narcosis studies, it was also able to recommend a limit of 60m as being the maximum depth at which compressed-air divers could operate safely. Sports divers also recognised his contributions, and he was made an honorary member of the Cave Diving Group.

Another discovery for which Jack Haldane was to receive very little credit, but which pleased him greatly, was that oxygen at 5-6 bar of pressure tasted rather like ginger beer! His father John Scott Haldane died aged 75, his lungs ruined by years of breathing poisonous gases. He passed away with a look of intense interest on his face as though he was taking part in some crucial experiment in physiology which had to be carefully monitored.
Perhaps his greatest legacy, his son Jack, died of cancer in 1964, having written some 23 books and more than 400 scientific papers during his distinguished career.
A diver to the end, he said that his one regret was not to have been able to walk to France - on the sea bottom!