A nearly 20-mile underground relic of Nazi tunnels, shafts, underground railway stations and combat facilities in Poland can now be toured after being transformed into an army museum. The subterranean city, called Festungsfront Oder-Warthe-Bogen, or the Ostwall, located in the serene countryside around the small Polish village of Pniewo, was abandoned in 1945. It is now a dark tourism destination, with 19 miles of eerie tunnels open for people to explore in the Mi?dzyrzecz Fortified Region Museum. “The Nazis planned this complex for long-term stay for soldiers, so everything is constructed to make it more habitable,” Mikolaj Wiktorowski, a guide...