A soldier competing in a gruelling race fell nearly 500ft (150m) after slipping on a steep descent in Snowdonia, an inquest has heard.
It was the first recorded death during a race in north-west Wales
Sgt Paul Upton, 37, a father-of-two, had been running for three hours in the Welsh 1,000m Peaks Race.
The Parachute Regiment soldier, from Northwich, Cheshire, died from head injuries after he landed on rocks.
Recording an accidental death verdict at Caernarfon, the coroner said the death in June was "amazing bad luck".
Nicola Jones, deputy coroner for north-west Wales, said Sgt Upton lost his footing descending the mountain.
"One small slip could have, and did have, fatal and catastrophic consequences," she said.
At the inquest Major David O'Brien, who was running alongside Sgt Upton, relived the moments before his death and his 40-minute battle to try to save him.
Major O'Brien described how he shouted a warning to the runners behind him after carefully negotiating a steep part of Carnedd Llewelyn.
He described how he then saw his colleague "flew past him" and saw him come to rest of the bottom of a crag area.
"I went into emergency mode and thought of a quick way to get to him which was straight down the crags, which I did and reached him"
Maj O'Brien told the inquest that Sgt Upton, who was based at RAF Brize Norton, was exceptionally fit and one of the race leaders.
He said: "I shouted at two people behind me to call an ambulance.
"As I approached him I was calling his name, saying 'Allright Paul', but there was no response.
"As I got to him I heard air move from his chest but there was no response.
"He wasn't breathing. I was aware he had bad injuries to the back of his head.
"He had a weak, rapid pulse and I went into mouth-to-mouth resuscitation and tried to stop the flow of blood."
The 22-mile race, which attracts the Army and running clubs from across the country, began at 0730 BST and the accident happened approximately three hours into it.
Sgt Upton, a divorcee who had become engaged and planned to remarry, was airlifted to Ysbyty Gwynedd hospital in Bangor by RAF helicopter, but was already dead.
The coroner said his death, caused by a skull fracture and brain haemorrhage, was the first fatality during a north-west Wales race since records began.