MRU

The Międzyrzecz Fortification Region (German: 'Ostwall or Festungsfront im Oder-Warthe Bogen', Polish: Międzyrzecki Rejon Umocniony) is a fortified military defence line in Western Poland, between Oder and Warta rivers.
Built in 1934-1938, it was the most technologically advanced fortification system of Nazi Germany, and remains one of the largest and the most interesting systems of this type in the world today. It consists of around 100 concrete defence structures partially interconnected by a network of underground tunnels. Some of the forts and tunnels are available for visiting.
The most interesting part is the central section, which begins in the south with the so-called Boryszyn Loop near the village of Boryszyn and extends about 12 km to the north. In the central section the bunkers are interconnected with an underground system of tunnels, 32 kilometres long and up to 40 metres deep. In the underground system there are also railway stations, work shops, engine rooms and barracks.
It is also the largest European underground bats refuge, giving shelter to some 32,000 bats of 12 species in the wintertime.