An enigmatic underground Nazi structure, left unfinished, continues to baffle historians since its purpose remains unknown.

In the final phase of World War II, the Third Reich initiated the construction of an expansive underground bunker using Jewish slave labor, in then-Germany, under what is called Project Riese.

Tragically, 5,000 workers died after they were brought from the nearby Gross-Rosen concentration camp to begin work in 1943.

Riese means Giant in German – a code name which represents the Nazi’s ambition with the project.

But the cavernous tunnels in modern southwestern Poland are now harmless and have become a tourist destination.

There are seven underground complexes that make up Project Riese in Poland’s Owl Mountains – with the City of Osówka just one of the locations.

Zdzisław Łazanowski, a guide at the Osówka complex, informed The Sun that this project might have been intended as a clandestine future headquarters for the Reich’s notorious leader.

He said: “Our best idea is that it was meant to be the headquarters of the army… and also the headquarters for the main person in the Third Reich – Adolf Hitler.”

But as the allies advanced on Germany, the purpose of Riese changed and the underground tunnels were used to build weapons.

None of the complexes were ever fully completed by the Nazis and Łazanowski said they haven’t all been explored.

He said: “We’ve explored eight and a half kilometres, but we know about two places where they [tunnels] were destroyed.

“There are lots of loose rocks and we need to dig about two metres, through a lot of loose rocks, to get to the tunnels.”

Nazi papers found in Prague following the war say that Riese needed another year of work by slave labourers to finish it.

Łazanowski said: “When we host our guests from all the world, we tell them that we are now at a building site, and we can see the the

the situation that was left when the Germans left.”

Osówka was almost completed and has railroads, mess halls, and power generators built in an underground grid of 1.1miles (1.8km) long.

There is also the bones of a concrete hall 10 metres high with a shaft for an elevator to service 16 planned floors.

Another bizarre structure is a concrete monolith with dozens of pipes, drains, and culverts, buried into the rock at least 14.7ft (4.5m).

There is also a unconnected shaft that contains a dam and hydraulic equipment – but for an unknown purpose.

Other sites in the complex would have hosted underground weapons factories – had they been finished.

There’s even a testing area where the Nazis would have worked on developing the V1 and V2 rockets.

Łazanowski said Riese would have been built into the mountain to protect it from bombers – similar to America’s Cheyenne Mountain Complex.

Some believe the tunnels are also hiding the fabled Nazi gold train.

The legend comes from a Pole who worked in the mine after the end of the war, according to Łazanowski.

Germans living in the area told the Pole that one train had collected all the precious items from the neighbouring villages and gone into the underground complex to hide from the advancing Soviets.

The Pole became so obsessed by the story that he rode through the tunnels on trains to try and find the treasure.

Łazanowski said they had not found the gold train inside the tunnels.

“But there are documents that say in the middle of January 1945 the Germans moved their money from the bank in Wroclaw… but we don’t know if it was put on the train.”

A detailed letter was sent to Polish authorities last month claiming to finally reveal the alleged train’s long-lost location.

The anonymous tipster claimed to have uncovered train wagons hidden in a tunnel in the Owl Mountains.

Others think the tunnels could be the place where the Nazis stashed the Amber Room.

The Amber Room – also sometimes referred to as the eighth wonder of the world – was one of Russia’s most treasured artefacts until it was looted by the Nazis from its palace home near Saint Petersburg and lost forever.

It had been installed at the Berlin City Palace and gifted by Prussian King Frederick William I to the Russian Empire in 1716.

The tunnels are not thought to have been visited by Hitler, but his infamous architect and minister for weapons production Albert Speer did.

Łazanowski said the tunnels were an important historical site because they were a sad memorial to the people that built them.

Riese’s builders were prisoners from Gross-Rosen concentration camp including Jews, Poles, and POWs.

Łazanowski said: “Our aim is to show people who come here to visit this place, show them the whole view of the working site, of the achievements, of the the Germans completed the idea of this place.

“But we also mention, and that’s the big part of our job, to tell the visitors about the history of people who are building this place, not just the people who planted this place, but the people who were digging out the rocks and were doing this cruel work for the

to track.”

The Red Army stormed through the Owl Mountains on May 8, 1945 – a day after the Nazis abandoned Project Riese.

But the Russians didn’t use the site, instead looting anything that could be carried back to Russia like drills or power generators.

Inside the secret Nazi WW2 tunnels hidden deep beneath the Dutch countryside

A LOST city of underground bunkers built by the Nazis on the Dutch coast during the Second World War has been laid bare in these astonishing pictures.

The bunkers, tunnels, living quarters and stores run throughout the coastline near The Hague and run deep under the city itself.

Its builders named it the “string of pearls” and, with the aid of original German army blueprints, they are now being restored to their original condition by military enthusiasts and archaeologists so future generations can visit.

Many of the bunkers were re-discovered when sands facing the North Sea shifted for the first time since the end of the war.

They were constructed by legions of Dutch slave labourers and German army engineers as part of the Atlantic Wall Hitler constructed from Norway to the Bay of Biscay in France in a bid to thwart Allied invasion plans of his “Fortress Europe”.

Read more here.