A coroner has given a stark warning about a condition known as "raptures of the deep" after three divers succumbed to it and died.
Rescuers are frequently called to the quarry pool
He recorded verdicts of accidental death on the three who died in separate accidents from nitrogen
They were among 20 who have died at the Dorothea Quarry in the Nantlle Valley in recent years.
The coroner, Dewi Pritchard-Jones, said this was far too many.
The three inquests were held together on Thursday to help draw attention to the problem and prevent fewer tragedies at the flooded quarry, which is more than 110m deep in parts.
Nitrogen narcosis can occur in divers breathing compressed air in deep waters.
Also known as "raptures of the deep", nitrogen narcosis is one of the most serious conditions divers can face, along with "the bends".
It can affect decision-making ability, and cause unconsciousness.
"It (nitrogen narcosis) affects the ability to deal with emergencies and what can be a minor incident becomes major," said Mr Pritchard-Jones, sitting at Caernarfon.
"I think the most important thing that should be highlighted is the danger of nitrogen narcosis."
Around 20 divers have died in a decade after getting into difficulties at the old slate quarry, which has a network of flooded tunnels and sheer drops.
Six tragedies have happened in the past year alone.
The inquest heard how experienced diver Henry Le Coz, a 36-year-old furniture maker from Bath, vanished in the pool on a 65m descent in November 2001.
Diver Paul Kay said there had been many "near misses"
A police underwater search team was brought in and a remote-controlled robot found his body at a depth of more than 90m.
Mr Pritchard-Jones said there was no clear explanation for the death but it seemed Mr Le Coz had been affected by nitrogen narcosis.
Michael Gott, 52, a shop fitter from Glossop, Derbyshire, drowned the following month.
The inquest heard he had reached a depth of 49m, but he got into difficulties after returning to a ledge at 40 metres and had trouble with his breathing.
A coupling on a hose attached to his jacket was not properly secured.
Mr Gott might have experienced difficulty inflating the buoyancy jacket and he wore a heavy weight belt, the coroner said.
Began to sink
"It's important to make sure equipment is functioning properly. Had that jacket been connected properly it would have been capable of being used and Mr Gott would have been able to ascend," he explained.
Nitrogen narcosis could have explained why he had not dropped his weight belt when he began to sink, he added.
The third inquest was held on Jon Hepherd, 53, a manager, from Warrington, Greater Manchester.
He died last December after he got into difficulties in an underwater tunnel nearly 60m under water. His friend Lyndon Taylor had tried to save him.
Mr Taylor noticed that Mr Hepherd had no regulator in his mouth so he offered him his spare one.
"Unfortunately, he wouldn't take it and clenched his mouth shut," he told the inquest.
"Within a few seconds he became unconscious."
Despite attempts by the owner of Dorothea Quarry to block access to the site, keen divers continue to travel hundreds of miles to try their skills in its waters.
Numerous websites feature the quarry as a favourite destination, and talk of its risks and its appeal. One even carries an underwater map of the site.
Diver Paul Kay - an underwater photography expert from Bangor - has previously said the quarry should not be tackled by the inexperienced.
"It does attract a element who see sub-aqua diving as a macho thing," he said.
"The death toll at the quarry could be an awful lot higher, as there have been an awful lot of near misses and lucky escapes."