Switzerland is a neutral nation but has more nuclear bunkers than any country in the world

A tunnel leading from the entrance of the Sonnenberg nuclear bunker in Lucerne, Switzerland, leads to a complex deep underground. The bunker is the world's largest underground nuclear bunker, constructed to house 20,000 people. Rob Schmitz/NPR hide caption

LUCERNE, Switzerland — From the outside, the Sonnenberg nuclear bunker looks innocent enough: its entrance is in the middle of a children's playground. But past the seesaws and swing sets, carved into a hill, is a thick blast-proof door that, when opened, exposes a narrow, well-lit tunnel that gently descends as far as the eye can see.

Sonnenberg Bunker guide Zora Schelbert walks slowly down the tunnel toward the main structure of the bunker, built in the 1970s, during the Cold War, to house 20,000 people in the event of a nuclear war.

"Back then, 20,000 people constituted a third of our city's inhabitants," says Schelbert. "So every third person in the city of Lucerne was once supposed to be sheltered in this bunker."

The entrance to the nuclear bunker underneath the University of Bern's History Department building. Rob Schmitz/NPR hide caption

If a nuclear war between the U.S. and the Soviet Union were to happen, Swiss officials reasoned at the time, Switzerland — an historically neutral nation — would likely not suffer a direct hit. But it would be in the path of nuclear fallout from its non-neutral neighbors. So they supplied diesel-powered generators that had enough fuel to power this bunker for two weeks.

"So after two weeks, no more food, no more water, no more power," says Schelbert. "And today, civil defense also says that the duration of stay depends on what the problem is outside. And as we can see currently in Ukraine or Israel, this makes total sense because as long as it's a conventional war, people seek shelter temporarily and then you can leave again."

Instead of scrapping its nationwide network of bomb shelters after the Cold War ended, Switzerland doubled down and kept building them, guaranteeing every one of its 9 million citizens a bomb shelter space, just in case. The country has more than 370,000 nuclear bomb shelters.