On the face of it, the quaint Polish village of Pniewo is a serene haven boasting exquisite countryside with patches of yellow crops and peaceful forest areas — but beneath the surface lies something far more sinister. A sprawling 20-mile maze of tunnels and shafts are hidden away, holding networks of underground railway stations and combat facilities that point to a harrowing time in human history.

Welcome to Ostwall, a fortified subterranean complex constructed by the Nazi regime that was abandoned when World War II ended back in 1945. It would then be taken over by so-called ‘Bunker People’ in the 1980s and 1990s, a subculture that would host some rather wild events there, namely weddings and even jam-packed raves. But these days only bats can be seen dancing throughout the hollow time capsule, with around 40,000 of them taking refuge in the darkness.

Ostwall is nestled between the Oder and Warta rivers near Międzyrzecz, and was Nazi Germany’s most technologically advanced fortification system, and even today stands as one of the world’s largest.

Adolf Hitler famously visited the facility during construction, which originally started in 1934 and would conclude a decade later, towards the end of 1935.

But amidst the war, the Soviet Army would reach the defence line during the course of the Vistula-Oder Offensive in January 1945 before the Germans could deploy troops, taking just three days.

There were workshops, storage spaces, social areas and barracks large enough to house 24,000 soldiers. Train tracks would then be built in the main traffic space to use as an underground road to connect it all together.

The area was highly suspected of being a hiding place for Nazi treasures and stolen artworks, though none are on record to have been discovered.

It has since been given a new lease of life as a dark tourism destination in the 21st century, with 19 miles of the tunnels open to explore.

If you ever wanted to visit a part of Nazi Germany history then you can today thanks to The MRU Museum and Fortifications in Pniewo which runs year-round underground tours.

Then there’s a second access point at the Boryszyn Loop where you can book either a walking or biking tour.