Festungsfront Oder-Warthe-Bogen
From Wikipedia: The Festungsfront Oder-Warthe-Bogen (Fortified Front Oder-Warthe-Bogen), also called the Festung im Oder-Warthe-Bogen or Ostwall (East Wall), and in Polish the Międzyrzecki Rejon Umocniony (Międzyrzecz Fortification Region), was a fortified military defence line of Nazi Germany between the Oder and Warta rivers. Built in 1934–38, 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.