The Viet Cong’s Cu Chi tunnels continue to inspire modern armed resistance. Aside from Gaza militants, separatists in India’s Jammu and Kashmir region are reportedly building underground bunkers in thick forests and elevated ridges in the Kulgam district.

Hidden bunkers pose a threat to Indian security forces who could be ambushed by “terrorists” using it for cover.

Indian troops stumbled on one of the secret bunkers during a firefight with two terrorists early this month. Security personnel thereafter found a few more of these hideouts, The Hindu reports.

Meanwhile, beneath the small Polish village of Pniewo lie bigger bunkers that were supposed to house Nazi troops.

Built in 1931 on the orders of Germany’s then leader, Adolf Hitler, the Ostwall, as the underground fortification was called, has a maze of tunnels, underground railway stations, combat facilities, and massive shafts throughout its 20-mile length, CNN reports.

Ostwall is now a tourist attraction that shows visitors the military engineering feat that was abandoned by the Nazis at the start of World War II.

Aside from being temporarily used for raves and weddings by the so-called Bunker People who inhabited it in the 1980s and 1990s, and by tourists, no one lives in the underground facility except for migratory bats.

With as many as 40,000 bats arriving annually from across Central Europe each fall, the Ostwall has become one of Europe’s largest colonies for the nocturnal mammals, according to CNN.