The Ostwall Fortification – Pniewo, Poland - Atlas Obscura
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Pniewo, Poland

The Ostwall Fortification

What was once an underground Nazi city is now Europe's largest bat reserve. 

When the Nazis built the imposting Ostwall Fortification in the mid-1930s the intention was to create a defense against invading Russian forces, but they clearly could not have foreseen the invasion of bats that would overtake the tunnels once they were abandoned.   

The Ostwall Fortification, otherwise known as the tongue twisting “Festungsfront Oder-Warthe-Bogen,” was constructed as a massive self-sufficient bunker defense system that was connected by almost 25 miles of tunnels and underground support chambers. The underground complex connected around a hundred pillbox bunkers creating one of the largest and most extensive defense lines Nazi Germany ever created. It was estimated that at full capacity and operation the system could house 24,000 soldiers comfortably. The bunker system was seen as a bit of a revolutionary endeavor at the time, both in terms of efficiency and efficacy. However when the attack on the base actually came, there were just 1,000 soldiers manning the massive defense. The Nazi’s impressive defensive juggernaut fell in just three days.

After the Nazi’s fled from their bunker city the site was essentially abandoned by humans but became home to some new tenants: tens of thousands of bats! Attracted to the tunnels thanks to the dark, temperate atmosphere and shelter for their winter hibernation, the flying mammals took to the tunnels in droves.

Today the tunnel complex has become the largest man-made bat reserve in Europe, holding around 37,000 bats during the packed winters during which tourists are not allowed in the tunnels. Many portions of the tunnels are open to visitors at other times of the year, but during bat season the Ostwall Fortification provides a safe space for the animals to rest defended from the outside world. The Nazis could only have wished it would’ve been so effective for them.