Two French tourists diving on Australia's Great Barrier Reef both died of heart attacks in close succession.
A 76-year-old man and 74-year-old woman were diving with a group of 21 other elderly French tourists when the tragedy occurred.
The weather and sea conditions were both deemed unrelated to the deaths.
"It just appears to be the perfect storm: two events occurring at the same time," said the Association of Marine Park Tourism Operators executive director Col McKenzie.
"We're dumbfounded. In 35 years, I've never seen a similar event."
It has been revealed that both victims had declared pre-existing medical conditions in paperwork filed before they went into the water.
Both the man and woman were found by lookouts on the beach near the diving area, and attempts to resuscitate the victims proved unsuccessful.
While the official cause of death is yet to be determined, the incident is not thought to be suspicious.