Something is hidden in the thick pine and beech forest. Just outside ?winouj?cie, a resort town on Poland’s Baltic coast, Wolin Island greets you with endless coastal dunes and seemingly untouched nature. But there’s a darker side to this idyllic-looking escape. Its sandy soil conceals an enormous, once top-secret bunker city. The complex, known today as Wolin Underground City, and originally called Vineta Battery, was built by the Nazis in 1936 as a fortress to guard the coast. Later, it became a nuclear command center for the Polish army. At one point during the Cold War, it was the center...