Number of ‘preppers’ grows amid heated rhetoric between Donald Trump and Kim Jong Un, but majority of country doesn’t expect war with North Korea.

As he headed for a private bunker beneath a mountain in suburban Seoul, Woo Seung-yep ran through his mental checklist: canned food, chlorine bleach, radio, gas mask. Since quitting his job as an information technology manager five years ago, Mr. Woo has been preparing for war.

Most of South Korea’s 51 million people tend to shrug off the North’s missile launches and nuclear tests—they have lived with the threat for years on the divided peninsula. Some 58% believe there is no possibility that North Korea would cause a war, according to a Gallup Korea survey released this month, up from 53% in 2014. Others aren’t taking any chances. Known as “preppers,” these urban-disaster specialists keep survival bags laden with essentials that they can grab at a moment’s notice as they flee to one of the country’s many shelters. As Pyongyang ramps up its weapons tests and threatens attacks on the U.S., preppers such as Mr. Woo say more are joining their ranks.

In other recent conversations, preppers discussed which flashlights could function in the event of an electromagnetic pulse attack, and compared the performance of portable water purifiers.

The movement has echoes of the Cold War, when some American families fashioned their own bunkers or fallout shelters.

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