Warning: This article contains reference to depression and suicide.
Billy Graham has seen it all in boxing.
The 68-year-old made his professional boxing debut as a teenager, but is most famous for steering Ricky 'The Hitman' Hatton through his glory years.
As he watches from ringside, he sees where the punches land, but also the innermost thoughts behind them.
"Fighters might say they don't want to hurt their opponent, but let me tell you, when you're in there, you do," he says. "You absolutely do.
"You want to knock them out, you want to keep hitting them until they drop, so they'll stop hitting you and you can get out of that hellhole.
"That's the reality."
It would be easy to presume that delivering a knockout punch is a moment of pure ecstasy, the kind of sensation footballers experience when scoring a goal.
But boxing is not football and knocking someone out is not like scoring a goal.
The knockout is brutal, final, irreversible - the fate all fighters dread and to which none are immune.
I have spent months examining that unique moment for a book, speaking to those on either end of sport’s most compelling, stark division of victor and vanquished.
What I found was surprising.
Deontay Wilder knocked down Bermane Stiverne three times in the first round of their November 2017 fight
For some fighters their ability to deliver a knockout blow is a defining characteristic, part of their DNA.
David Haye and Deontay Wilder are two such men. Between them they have won 71 fights, 68 of them by knockout. This is how Haye describes the knockout moment.
"It's a beautiful feeling," he says. "We love the battle but to end the conflict in one fell swoop, the buzz… you can't compete with that.
"I've never found any high that can come close to that."
Wilder takes it a step further.
"You feel like a god. You feel very powerful," says the American.
"It's an indescribable feeling. The aftermath of it, it intensifies your feeling."
For fighters like them, boxing is a religion and the knockout is their idol.
For others though, the feelings are more ambivalent.
"It's a mixture of emotions and I don't think until you wake up the next day you understand actually what you've done."
That is how Carl Froch describes his feelings after knocking out George Groves in front of 80,000 people at Wembley Stadium in 2014, with what proved to be the final punch he threw as a professional boxer.
The build-up to the fight had been marked by animosity, with Groves - who had suffered a controversial loss in the pair’s first meeting six months previously - antagonising Froch at every opportunity.
But Froch’s satisfaction was professional, rather than personal, as a huge right hand crumpled Groves to the canvas.
"Your mind's in a mad space," he says. "But once it was stopped, I didn't get too emotional. I was that much in the zone, it was just seek and destroy and when the knockout came I just kind of knew it was coming."
If there is one emotion, it is relief. That is the most common emotion mentioned by fighters. Relief that it's over, that it's not them who now has to suffer the pain and humiliation of defeat, of failure.
It was exactly this feeling that Tony Bellew experienced when he knocked out Ilunga Makabu at Goodison Park to become WBC cruiserweight world champion.
"I just kneeled down and cried," he says.
"Cried with relief at the fact that everything I'd been saying all these years had come true. It was just the greatest relief of my life.
"It just validated everything I'd been saying, that I was gonna be a world champion. It was the ultimate goal. I'd reached the ultimate goal."
But sometimes that initial surge of relief morphs into something else. Sometimes the relief at seeing an opponent fall, knowing that the fight is over, quickly metamorphoses into a cold and icy fear, born of the realisation that the opponent isn't moving.
Jamie Moore experienced this rising dread after knocking out Matthew Macklin in the 10th round of a full-blooded British light-middleweight title bout in 2006.
Macklin left the ring on a stretcher, but was released from a local hospital after precautionary checks. Moore cancelled his victory party to visit Macklin in hospital.
"This sport is beautiful and brutal in equal measure because you want to inflict pain on your opponent because that's the only way you're gonna win," says Moore.
"But when you do it to that extent, then that scares you. Everyone wants to win but nobody wants to go to that extent.
"So you're trying to find that middle ground. But when you push it to that end of it, it's a scary feeling."
Billy Graham was Macklin’s trainer that night. As he knows, that middle ground is a mirage.
It doesn't exist. It can't. Not in a sport like boxing.
Any empathy has to wait until the contest is over. But when it comes, it comes easily. Because every winner knows he is only ever a split-second away from swapping places.
Francis Ngannou, an MMA champion, lost a controversial split decision against Tyson Fury on his professional boxing debut, before being knocked out by Anthony Joshua
In early March, Francis Ngannou was knocked out by Anthony Joshua.
Ngannou's legs folded underneath him, more than 19 stone of bone, sinew and muscle going limp. His limbs poured on to the canvas in languid fashion before settling in an almost perfect symmetry; legs stretched out straight in front of him, arms resting neatly at his side, his entire frame flat on its back.
Unconscious.
He told us later that he didn't feel or a thing.
Some found that shocking but that's how it is when a fighter gets knocked out cold. The violence of the blow to the brain is so great that there is no chance of a person being able to recall it.
Hatton was knocked out by Manny Pacquiao in equally brutal fashion in a Las Vegas super-fight in May 2009.
"I was knocked out cold and then after a couple of minutes you come around," he says.
"You wake up and then you realise you're in the ring and that you've been in a fight and you go back to the changing rooms after and then you talk to your coach and ask: 'So what was it? What was the punch that caught me">