Samarn Gunan had been returning from inside the waterlogged cave when his oxygen supplies ran out, officials said.
The former Thai Navy Seal had been working as a volunteer during an overnight mission in which he had been delivering oxygen canisters, according to authorities.
On his way back he lost consciousness, Thai Seal commander Apakorn Yookongkaew said, adding that a friend had tried to help bring him out.
But even though we have lost one man, we still have faith to carry out our work.
The diver’s death highlights the perils of the operation to bring out the young boys and their 25-year-old coach from inside the Tham Luang Cave system.
Thai authorities are racing to pump out water from the flooded cave where the group have been trapped since 23 June.
After the euphoria of finding the boys alive, the practicalities of getting them out are focusing minds, says Sky’s Alex Rossi.
Rescuers have managed to reduce water levels in the cave by around 40% but monsoon rains are expected on Saturday.
The boys, aged 11 to 16, and their coach were found on Monday, nine days after they went missing following a football game.
Parts of the passageway leading to where the group were found are still flooded all the way to the ceiling – leaving diving out of the cave the only option for survival.
Officials have said not all 13 of those trapped may be extracted at the same time depending on their condition, and the boys have been practising wearing diving masks and breathing.
They are not thought to have attempted any practice dives.
The Thai Navy Seals are in charge of extraction plans, with the help of dive teams from Britain, America, Australia and Asia.
News Source SkyNews