In rural Poland near to the town of Międzyrzecz lies an impressive underground fortification built by the Nazis in the 1930s. Although it soon became redundant with regards to its original purpose, the Festungsfront Oder-Warthe-Bogen (or 'Ostwall') has over time taken on a new purpose as a hibernation location for bats.
Check out this gallery to learn about how these hidden Nazi tunnels became a favorite hideout.