Międzyrzecz Fortified Region
A complex of wartime fortifications, the longest military defensive system in the world, was ordered by Hitler and built between 1934 and 1938. The bunkers with reinforced steel walls are interconnected by underground corridors and form what is believed to be the longest defensive system in the world. Guided tours through the network of underground tunnels are available. As a result of the microclimate that has developed, the fortress ruins have become a bat reserve called "Nietoperek" which have become a refuge for more than 30,000 bats.