Nowadays we associate The Fortified Front of Odra-Warta Rivers (Międzyrzecki Rejon Umocniony – MRU) mainly with the biggest in Central Europe bat reserve. Thanks to the constant moisture and temperature (about 8-10°C) about 35 000 bats of 12 species overwinter here from October till April.
But what happened here before?
After the World War I Germany was military limited and on the strength of the Treaty of Versailles it lost Alsace and Lorraine for the benefit of France.
Although it was forbidden, Germany wanted to launch an attack on France to get Alsace and Lorraine back. However Poland (stronger than Germany at that time) formed an alliance with France, so the plan started to be too dangerous. If Poland helped France, Germany would have to fight on two fronts.
That’s why Germans started to build some fortifications at the end of the 1920s. They built them along the east border of Germany (there is Poland now). At the beginning the built only water barriers (e.g dams or movables bridges) and small bunkers. Than in 1933 Hitler came into power and decided to build 3 following bunker systems: The Pomeranian Wall (Wał Pomorski) in the north; The Oder Position (Umocnienia Odry) in the south; The Fortified Front of Odra-Warta Rivers (Międzyrzecki Rejon Umocniony – MRU) in the middle (you can visit it here).
The underground tunnels (about 30 km in total) interconnect 26 bunkers. The underground system was supposed to be an independent and a self-sufficient “city” with railway stations, machinery rooms / engine rooms (electricity), barracks, workshops, ammunition stores, toilets, bathrooms, shafts (10-40 m deep), stairwells, etc.
In 1938 the building was stopped. Germany was stronger than Poland and Hitler planned the World War II