Adam Walker, from Nottingham, was swimming off the coast of New Zealand back in 2014, when the predator was seen lurking in the depths below.
At the time, he was told that sharks weren’t around and he would be able to enjoy a fear-free dip in the Cook Strait.
But luckily for old Adam, a pod of dolphins popped up to save the day, protecting him from harm.
In footage shared to YouTube, Adam can be seen in the middle of his 16-mile challenge, as a support boat travels alongside him.
Then suddenly a group of dolphins pop up out of the water, swimming around him in circles.
A couple are even seen jumping into the air just beside him.
Writing about it in a Facebook post at the time, Adam said it was an unbelievable experience and felt as though the mammals were keeping him safe during his swim.
“Swimming with dolphins on my 6th of Oceans 7 swims ‘Cook Strait’ open water swimming doesn’t get any better than this!! Dream come true!” he wrote.
“I’d like to think they were protecting me and guiding me home. This swim will stay with me forever.”
However, Adam revealed that he didn’t even know there was a shark anywhere near him until he got out.
He had been told by a friend before getting in the water that sharks wouldn’t be an issue, so he wasn’t best pleased when he discovered the truth.
Speaking to the Marlborough Express, Adam said that he ‘had a go at him when I got out’.
Dolphins are often targeted by sharks, and rely on each other for protection.
The animals use their large snouts to prod sharks if they come in to attack, beating their underbelly or gills.
The Whale and Dolphin Conservation (WDC) organization said: “The main advantage dolphins have against shark attacks is safety in numbers; they stick together in pods and defend one another from a shark’s attack by chasing and ramming it.
“Dolphins are able to protect vulnerable members of their pods and extended families such as young dolphins and injured or sick dolphins.”
Adam completed his swim in 8 hours and 36 minutes.
It was the sixth in a series of grueling swimming challenges he set himself to try and raise some money and awareness for the WDC.