It was, said Sir Winston Churchill, “the greatest adventure story to come out of the war”, an epic underwater escape right under the noses of the Nazis that led all the way to Rosyth.

Polish submarine ORP Orzeł etched its name in wartime glory when the crew escaped internment in Estonia, then navigated their sub without charts and scarcely any defences through Nazi and Soviet controlled waters.

On a journey fraught with danger, they used lighthouses as their guides and brushed off enemy attack. Then, in the safety of British waters but without means of identifying itself, came the nightmare of friendly fire.

The crew went on serve as part of the Royal Navy, based at the Fife dockyard – where they had received a triumphant welcome - and carrying out a number of successful missions.

But in early summer of 1940, the Orzeł’s luck finally ran out. Having set off from Rosyth for another secret task, it vanished without trace somewhere in the North Sea.

Now renewed efforts are under way to find the long-lost submarine and bring to a conclusion one of the great mysteries of the Second World War.

The Herald: Pitor Michalik and the hunt for the missing subPitor Michalik and the hunt for the missing sub (Image: Piotr Michalik)

Earlier this week, a crew of six set off on board a specially equipped search vessel from the port of Ijmuiden in the Netherlands, for what will be the tenth expedition aimed at finding the remains of a submarine hailed as a symbol of Polish fighting spirit and patriotism.

Piotr Michalik, historical researcher and member of the Santi Find the Eagle team, said he is hopeful the new search will finally locate the resting place of the Orzeł both so it can be designated as an official war grave and to bring comfort to the crew’s relatives.

“It is really important to the families of the crew,” he said. “By doing this we can close a chapter in history and help people understand what happened to their relatives.

“It’s very personal for them to know that it is recognised as a war grave.”

The team’s previous searches have pinpointed the final resting places of more than 400 sunken vessels including HMS L10, a First World War submarine whose position was previously unknown.

The Herald: The mission to find the sub is a passion for the crew who have set offThe mission to find the sub is a passion for the crew who have set off (Image: Newsquest)

While the team’s search in 2017 uncovered the wreck of HMS Narwhal, which disappeared in July 1940 with the loss of 59 lives. It was found around 225km east of the Scottish coast.

The hunt for the Orzeł – Polish for "Eagle" - has spanned huge areas of the North Sea, with the search team taking their lead from the theory that the sub may have been accidentally sunk by the British planes in a friendly fire incident.

However, they say new information now leads them to suspect that theory is flawed.

Instead, they believe the Orzeł may have been operating in a particular area around 100 miles from the coast of the Netherlands.

It’s emerged that another British submarine, HMS Taku, which set out on patrol after the Orzeł, was in the area known as the A3 zone when an explosion was recorded, possibly linked to the Polish submarine.

The team also think that because no radio signals were received from the sub after she left Rosyth on May 23, 1940, it was probably lost soon after setting off – which would tie in with her being in that vicinity.

“Anecdotes circulating after the disappearance of ORP Orzeł suggest that it may have also strayed off course during its last mission. These are just anecdotes, but they support our new area of research,” he added.

The Herald: Previous expeditions and search areasPrevious expeditions and search areas (Image: Newsquest)

Preparations for the search have involved scouring naval and wartime archives in Scotland and London to establish Allied submarine movements at the time, and records held by the Sea War Museum in Jutland.

Researchers have also conducted interviews with North Sea fishing boat crews who have provided details of incidents involving nets being hooked on underwater obstacles and their own observations.

The Orzeł became a symbol of immense Polish pride following its daring escape. The sub has been commemorated in medals and monuments, while the story of how it defied the enemy only to be lost has been told in several movies.

Built amid mounting fears of a looming Nazi threat, it was one of the most technically advanced submarines of its times and followed a nationwide ‘crowdfunding’ effort to help pay for it.

It was delivered by its Dutch builders to the Polish navy just months before war broke out but was soon attacked and damaged.

The 1,473-ton sub made for the neutral port of Tallinn in Estonia, where Nazi pressure led to the sub’s crew being interned, their charts and navigational equipment seized and almost all weapons dismantled.

In a remarkable act of bravery, the crew overpowered the Estonian guards and, with machine-gun fire and shells peppering the sub’s conning tower, staged their dramatic getaway - despite running aground on the way.

Against the odds and with depth charges being dropped from above, the crew navigated silently along the Baltic coastline, even daring to halt off the fringes of Sweden to allow two Estonian guards caught up in the escape  to disembark along with clothes, money and food to help them return home.

During its remarkable 44-day voyage to Rosyth, the sub spent days submerged on the bottom of the sea hiding from the enemy and flew a faked Swedish ensign to trick German warships.

As it edged closer to the UK, however, it had to dodge friendly fire from Allied vessels who mistook it for a German sub.

“It was a remarkable escape, and then they went on to do missions out of Rosyth - the Eagle then spent most of her life in the Royal Navy based at Rosyth,” added Mr Michalik, a former banker who says he has poured all his earnings into the hunt for the Eagle.

“On one mission, the sub sank a troopship carrying German soldiers on their way to Norway.

“The Allies didn’t know that Germany was about to invade. It was only when the Orzeł sank this ship that they found all these Germans in lifeboats and realised they were on their way to invade.”

The Orzeł was lost with 54 crew aboard, among them some French and British sailors. Efforts are currently under way to raise funds to create a memorial to the submarine and its crew in Rosyth.

The search team of six includes specialist hydrographers, divers and historical researchers and will last around a week.

“This is not a hobby, it’s a passion,” added Mr Michalik. “The Orzel, the Eagle, the legendary lost WW2 Polish submarine sailing in the Royal Navy, is not forgotten.

“We are as determined as ever to find her location and solve the mystery of her disappearance.”

To follow the search, go to www.santifindtheeagle.com