A former head teacher died after wrongly believing he had run out of oxygen while diving in a flooded quarry, an inquest has heard.
But the direct cause of death remained a mystery after a pathologist said it could not be ascertained.
Christopher Lindup, 58, from Bristol, got into difficulties during a night dive at Vobster Quay Diving Centre, Somerset, in September 2004.
Coroner Tony Williams recorded a verdict of death by misadventure.
The inquest was told Mr Lindup believed he had run out of oxygen and aborted the dive but seconds after returning to the lake surface he disappeared under the water.
Mr Lindup previously worked as a head teacher at both Lawrence Weston School and Merrywood School in Bristol - he retired in 2000.
Diving colleagues said he was an "experienced and competent" diver who had completed more than 60 dives.
The inquest in Wells, Somerset, heard Mr Lindup and regular diving companions Clive Finn and Gary Ward visited the quarry on the evening of 30 September for a routine night dive.
Mr Finn, a diver with more than 10 years' experience, and Mr Lindup intended to complete a 45 minute dive, looking at old wrecks and buildings submerged when the quarry had been flooded.
Halfway through the dive Mr Lindup signalled he had run out of oxygen.
Mr Finn attached his own oxygen supply to Mr Lindup and they swam to the surface.
Mr Ward, who had not been diving with them because of a cold, threw a rescue buoy to them.
Mr Finn said he let go of Mr Lindup who was still conscious and talking, so he could grab the buoy.
He said: "I swam out a few feet to get the buoy and as I turned back to Chris he just wasn't there, there was no screaming or splashing.
"What's really upsetting is that we were only 10ft from the shore."
After two attempts Mr Finn managed to bring him to the surface. Attempts to resuscitate him failed and he was taken to Bath Royal United Hospital where he was pronounced dead.
Consultant pathologist Dr Hugh White, who carried out a post mortem examination on Mr Lindup, found that he died "following a period of immersion underwater".
He said there were "no classical signs of drowning" and ruled that the cause of death could not be ascertained.