An American tourist died of a heart attack after snorkeling in freezing waters at Iceland’s most popular diving site on Sunday.

The 65-year-old collapsed at the side of the Silfra fissure, where his tour guide performed CPR as other horrified tourists looked on.

The tourist, who has not been named, was flown to the National University Hospital in the nearby capital city of Reykjavik but died soon after.

The death is the eighth serious accident to occur at Silfra since 2010 and the fourth fatal one, according to local reports. The spectacular 90ft deep, 1,500ft long fissure has water so clear that divers can see up to 40ft away from them.

Park officials have raised concerns about the 50,000 people diving there each year as tourists find it hard to adjust to the cold water.

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An American tourist died of a heart attack after snorkeling in freezing waters at Iceland’s most popular diving site, the Silfra fissure, on Sunday. This photo was taken on the day of the fatal dive by Scot Hacker, who witnessed the scene and spoke with DailyMail.com about what he saw

The unidentified tourist was part of a tour of eight people who went for a lunchtime snorkel in water that hovers around 30 degrees Fahrenheit all year

Large crowds mean they have to wait for some time for their turn in dry suits, which constrict their blood flow, making an accident more likely.

The tourist was part of a tour of eight people who went for a lunchtime snorkel in water that hovers around 30 degrees Fahrenheit year round.

Scot Hacker, 51, who was diving with a different tour company, wrote on Facebook that he ‘watched a person die today, and am feeling shaken’.

Scot Hacker, 51, who was diving with a different tour company, wrote on Facebook that he ‘watched a person die today, and am feeling shaken’

Hacker told DailyMail.com that as he was getting out of the water he looked to his right to see a the man, who had a stocky build, having clear difficulties.

Hacker said: ‘The person was on the ground on their back. There was one person kneeling over them doing CPR and a group of five people standing close by.

‘A helicopter came in and we were asked to move back. Our guide shooed us out of the area. It was shocking and such a dark thing to happen after such an incredible experience.’

Hacker, an app developer from El Cerrito, California, added that there were a lot of tour groups at the fissure that day, meaning each group had to wait on a bench for their go.

The Silfra fissure is considered a bucket list activity by many. Reviews on Tripadvisor say that ‘words cannot begin to describe’ the beauty and say it’s ‘all worth it’.

The site is where the European and American tectonic plates meet, and the water that fills the fissure bubbles up from the center of the Earth.

‘Our guide shooed us out of the area. It was shocking and such a dark thing to happen after such an incredible experience,’ Hacker, pictured on the left at Silfra, said

The tourist was flown by helicopter from Thingvellir National Park to the National University Hospital in Reykjavik, 30 miles to the east, but died soon after

Safety has been a concern for some tourists at the site and one review on Tripadvisor said: ‘The dry suit is kinda scary and when they put the hood over your head you may have a panic attack as you feel tense with sense of suffocation’

Divers at the site, located in Thingvellir National Park, 30 miles east of the Reykjavík, must obtain a permit from park authorities. Most tour groups charge between $250 and $350 for the three hour trip, of which 30 minutes is spent underwater.

Safety has been a concern for some tourists and one review on Tripadvisor said: ‘The dry suit is kinda scary and when they put the hood over your head you may have a panic attack as you feel tense with sense of suffocation.’

An official at the Icelandic Coast Guard, which flew the tourist to the hospital on its helicopter, told DailyMail.com that first responders were nervous about some tour groups being too ‘gung ho’.

The official said: ‘There are so many tourists diving there and there is no infrastructure at Silfra.

‘If you are snorkeling you at least need to be able to swim – they will basically let anybody in.’

Einar Ásgeir Sæmundsson, the spokesman for Thingvellir National Park, said that the tourist became ‘dizzy’ as he was about to come out of the water.

He said: ‘What it seems like is that man suffered from a heart attack in the water.

‘He was snorkeling with his group and he was getting out of the water when he became ill.’

South Iceland police chief superintendent Oddur Árnason said that he was still waiting for the autopsy results to reveal the exact cause of death.

He said: ‘There appear to be indications that the person did not drown but there was an illness, a heart attack or something.

‘Police in Iceland have the duty to investigate accidental deaths whatever the reason. I find it unlikely it will turn into a criminal investigation but we have a duty to investigate.’

A spokesman for the State Department said: ‘We can confirm the death of a US citizen in Iceland on February 12, 2017.

‘We offer our sincerest condolences to their friends and family. We stand ready to provide all appropriate consular services.’