A UK graduate died after performing an underwater "party trick" at his brother's 21st birthday, an inquest has heard.
The 24-year-old Cambridge student, Dominic Hamlyn, became unresponsive while swimming in a pool at his family home in Kent, England's southeast last year in July.
The "fit and healthy" man swam two-and-a-half laps while holding his breath before he stopped moving.
He was taken to William Harvey Hospital where he died later that evening.
During the inquest last Thursday (local time), his neurosurgeon father Peter Hamlyn confirmed his son had often done the underwater challenge.
"It was an entirely routine thing which I had seen him do since he was a child," he told the hearing.
He added that his son was had been unresponsive "for seconds rather than minutes" before being pulled out of the pool.
A medical student at the party initially performed CPR before Peter Hamlyn took over until paramedics arrived at about 3.45am (local time).
Coroner Scott Matthewson said the graduate had a pre-existing heart condition and died from acute cardiac arrhythmia - known as "sudden adult death syndrome".
"Nobody could have, or will ever with any certainty, say what the medical cause was in Dominic's death," he added.
His parents wept and held each other's hands as the assistant coroner gave his ruling, which acknowledged the differences in medical opinion.
Hamlyn's father described his son as a "beautiful, beautiful boy".
"Why did he die?" he said.
"He was a superb athlete competing in both rugby and rowing at Cambridge. We are broken. If Dominic is to be remembered, it is as a hero and one of the world's helpers."
The family later issued a family statement, saying data showed at least 12 young people die of cardiac arrest each week in the UK, but their inquest experience suggested this was likely to be a gross underestimate.
"As a result, victim's families will continue to go unscreened and readily treatable warning signs missed," he said.
"If I were not a doctor and one with specialist expertise in sports medicine my family would number amongst them.
"In life, Dominic gave so much to others. It is up to us to ensure his legacy is not just a family immersed in our grief but that his loss brings awareness and change.
"It was not possible to save his life. It is possible, with the knowledge we have today, to save other young lives."