Mauerwald Bunker Complex in Mamerki, Poland

When Nazi Germany invaded the Soviet Union in 1941, Hitler had elaborate military bunker complexes built throughout Masuria (then part of Germany, now part of Poland) in order to keep a closer eye on the war effort. While Hitler and his inner circle based themselves at the infamous Wolf's Lair, the Nazis' Supreme Military Command was located at a separate site called "Mauerwald" (now Mamerki) just 20 kilometers to the northeast.

The full Mauerwald military complex consisted of nearly 250 separate buildings and bunkers constructed in the forest just west of Mauer Lake (now Mamry Lake), the majority of which remain abandoned and freely accessible in the woods. The heart of the compound, however, is now confined within the Mamerki Bunker Museum, which seeks to preserve the core bunker structures. The museum is complete with underground tunnels and exhibitions on the Nazi war effort, including full-size recreations of daily life in the bunkers and other scenes.

Adding to the location's lore, Mauerwald is also one of several locations theorized to be a possible hiding place of the Amber Room. Erich Koch, purportedly the last person to have known the room's fate, was brought to the site in the 1970s as part of the search, and ground-penetrating radar has also been used since 2016 to search for hidden chambers. Although the search has proved fruitless so far, the sheer size of the complex suggests that it may still have additional secrets to reveal.