Video report by ITV News Washington Correspondent Robert Moore
Eighteen months ago, Vernon Unsworth helped plan one of the most challenging cave rescues of all time.
But today he was in a dramatically different environment, heading into a Los Angeles court room to defend his reputation, he said: "I've come 5,000 miles just to get a verdict that it's not true."
The remarkable rescue of the 12 young Thai soccer players and their coach captivated the world.
The tech billionaire Elon Musk offered to build a mini-sub to rescue the trapped boys, an idea that Unsworth ridiculed as utterly impractical given the twisting passageways inside the flooded cave system.
Musk angrily responded at the time with this tweet: "Sorry, pedo guy, you really did ask for it."
He subsequently deleted it and apologised.
Elon Musk and his legal team will be claiming to the jury that in that infamous tweet describing Unsworth as a "pedo guy" was just intended as a casual insult, and was not meant to suggest that the acclaimed cave diver was an actual paedophile.
The problem of course is that Musk's Twitter profile is immense - he is nearly 30,000,000 followers.
So almost immediately after the rescue of the boys, Unsworth threatened legal action.
Speaking in August 2018, Mr Unsworth said: "I believe he's called me a paedophile... I think people realise what sort of guy he is."
Musk's role as the billionaire founder and driving force behind Tesla and Space X has given him a worldwide platform.
Famously, he even launched his own personal Tesla into space... but his stratus as a tech hero has been jeopardised by his controversial his use of social media.
Musk arrived at the court house this afternoon. The jury must now decide whether he defamed the character of Unsworth, whose skill in that uplifting rescue has been widely recognised in Britain and around the world.