An experienced caver died when he became overcome by noxious gases at a slate mine, an inquest has heard.
Will Smith, from Hinckley, Leicestershire, was with two friends at the site in Aber Las, near Glyn Ceiriog, Llangollen, when he got in to trouble on 23 August last year.
The hearing in Ruthin heard he fell down an embankment into deep water after running out of air.
Coroner John Gittins recorded an open conclusion on his death.
'No undue risks'
Mr Smith's friend D'arcy Foley told the inquest the group knew the air in the cave's chambers was poisonous with carbon dioxide and hydrogen sulphide and were using air tanks.
After he collapsed, Mr Smith's friends tried to put an air regulator in his mouth but his jaw was clamped tight.
The two made their way to the surface and raised the alarm. Mr Smith's body was recovered the next morning.
Checks on the group's equipment found it was all in working order.
A post-mortem examination failed to reveal any medical cause for Mr Smith's collapse and a site visit was deemed to be too dangerous.
Mr Gittins said there was no question of Mr Smith having taken any undue risks, adding: "I think it is simply a case of his being overcome by gases in that environment."