Elon Musk has gone on trial in the United States over an infamous "pedo guy" tweet he posted last year.
The billionaire posted the comment online during the dramatic rescue of a group of children stuck in a cave in Thailand.
In the tweet, which has since been deleted, Musk called British caver Vernon Unsworth a "pedo guy."
Unsworth had previously dismissed the Tesla founder's idea to use a submarine to assist with the rescue of 12 boys and their coach who were lost in the Tham Luong caves.
Unsworth has since sued the South African businessman, who has a net worth of around US$23 billion (NZ$35 billion), for defamation.
Musk posted the controversial tweet after Unsworth, who lives in Thailand and has detailed knowledge of the caves where the team was lost, made fun of Musk's idea, saying it "had absolutely no chance of working".
He also suggested Musk "stick his submarine where it hurts".
Later, in an email to a reporter from Buzzfeed, the Space X founder referred to Unsworth as a "child rapist".
Lawyers representing Unsworth, who is seeking punitive and compensatory damages, called Musk's tweet "vile and false", according to BBC News.
Musk has defended his tweet, saying he didn't mean Unsworth was literally a paedophile.
"Pedo guy was a common insult used in South Africa when I was growing up," he said in a court filing, according to BBC.
"It is synonymous with 'creepy old man' and is used to insult a person's appearance and demeanour, not accuse a person of paedophilia."
A jury for the case, which is being held at a federal courthouse in Los Angeles, was chosen on Tuesday (local time).
The case is scheduled to last around five days, with Musk expected to testify in his own defence.