3. Nazi concentration camp, Mauthausen, Austria
The secret weapons complex was found near Mauthausen-Gusen concentration camp in Austria Herwig Prammer/Reuters

A secret underground weapons stronghold, built by the Nazis to test new advanced weaponry has been uncovered in Austria.

The surprise finding was discovered underground near the town of Sankt Georgen an der Gusen, Austria, near the Bergkristall factory where the Messerschmitt Me 262 – the first operational jet-powered fighter – was invented.

The discovery of the site came after Andreas Sulzer, an Austrian documentary maker noticed a reference to the bunkers in the diary of Austrian physicist who was recruited by the Nazis.

Sulzer, alongside a team of historians, began to investigate the idea of the bunkers, receiving funding from several German broadcasters to document his discoveries.

Sulzer, one of the excavations' coordinators, spoke proudly of the discovery: "This was a gigantic industrial complex and most likely the biggest secret weapons production facility of the Third Reich."

He acquired aerial photographs from RAF plane that showed outlines of the complex's concrete structure.

Prior to their downfall, the Nazis went to great lengths to conceal the weapons facility, covering the entrances with thick layers of earth and slabs of granite.

The vast underground network of tunnels were built by inmates at the nearby Mauthausen-Gusen concentration camp. It is believed that an estimated 320,000 inmates died building the labyrinth of concrete tunnels and shafts.

It is believed to be the location of a secret weapons programme, led by SS General Hans Kammler. Kammler is thought to have lived on the site during the war. Rumours continue to surround his death, with some suggesting he was given a new identity by the US government in exchange for details on Nazi weapons research.

The deadly V-2 rocket which struck at London during the final months of the World War II was tested at the complex. It is also thought that scientists experimented with the use of radioactive material and chemical gas.

The new excavations have had a setback, after local authorities ordered them to be halted until Sulzer obtains a proper permit for the research of historical sites.