Harrowing true story of married couple who disappeared and were never seen again after being left behind on diving trip

This is the haunting true story of a couple who were accidentally left behind while on a diving trip, never to be seen again.

The horrific ordeal that American couple Eileen and Tom Lonergan are assumed to have went through even went on to inspire a film called Open Water.

It was back in 1998 when Eileen and Tom, from Baton Rouge, Louisiana, were on the trip of a lifetime in Australia.

The married couple were on their way back home to the States from Fiji after having served in the Peace Corps for a year, and decided to make a pit stop in Queensland for the chance to dive the world’s largest coral reef system.

With Eileen, 28, being an experienced scuba diver, it didn't take her long to convince her husband, 34, to join her in what they thought would be an experience they'd cherish for a very long time.

Alongside 24 other passengers, they boarded a boat called Outer Edge, manned by skipper Geoffrey 'Jack' Nairn, and travelled 60km off the coast to enjoy a day of diving.

As standard with diving trips, a headcount was done at the end of the day and the boat returned to the mainland.

Eileen and Tom Lonergan went on a diving trip in Australia, only to never be seen again (Queensland Police)

But what everyone onboard didn't at first realize was that Eileen and Tom weren't among them and were, in fact, left stranded out at sea.

And to make things even worse, it wasn't until two days after the dive when it was realized by Nairn that the pair had been left behind when he discovered a bag aboard containing their personal belongings, wallets, and passports.

A wide-spread air and sea search took place but, tragically, the couple were never found, leaving their family and friends in utter turmoil - especially after incorrect and hurtful rumors began to spread that Eileen and Tom were involved in a murder-suicide, or that they had staged their disappearance.

It wouldn't be until a month after their disappearance that signs of the couple began to emerge, with a wetsuit in Eileen's size being found washed up on the northern Queensland shore, as well as dive jackets, tanks and one of Eileen's fins.

But the most harrowing discovery came even more months down the line when a fisherman discovered a dive slate - an accessory used for making notes underwater - which had a chilling note scrawled on it.

The couple were accidentally left behind by the group they were diving with (Tom and Eileen Lonergan)

"To anyone who can help us: We have been abandoned on Agin court reef Reef 25 Jan 1998 03pm," the note read. "Please help us come to rescue us before we die. Help!!!"

The date and time written on the slate suggested Eileen and Tom managed to survive for around a day in the water before sadly losing their battle.

Reports over the years have stated that there was no evidence to suggest their lives came to a violent end, with the film Open Water depicting them as being victims of a shark attack.

Skipper Nairn was charged with manslaughter, with coroner Noel Nunan alleging that he was responsible for the deaths of Eileen and Tom.

Nairn was later found not guilty by a jury, though his company pleaded guilty to negligence and went out of business.