In the northwest of Warsaw, in a part called Babice, next to an existing fort, lay during the war a Transatlantic radio station (switchboard) which was used by the germans to contact their submarines in the Atlantic. The station was destroyed by the Germans in January 1945 when the Soviet Red Army forced them to leave Warsaw.
Current status: Demolished (2023).
Location: 52°15' 51.63" N, 20°52' 41.86" E
Get there: Car.
Follow up in books: Blair, Clay: Hitler’s U-Boat War: The Hunted: 1942-1945 (1996).
The ruins of the station remain about ten meters into the forest and are easy to find and there is also an information board on site.