CNN news 2017-05-11 AZUZ: Starting last year, North Korea stepped up its controversial nuclear program. There's been an increase in rhetoric, threatening speech there regarding the United States. An American aircraft carrier strike group is headed toward Korean waters following a North Korean missile test last week, and the communist country issued a warning that it would respond to any, quote, "reckless acts of aggression". Meanwhile, China, which borders to the north, is calling for calm. But what's the mood like across the other border, in South Korea? It's a U.S. ally, a potential target of the North. Are people there concerned about all this? PAULA HANCOCKS, cnn INTERNATIONAL CORRESPONDENT: Simulating an attack on a subway in Seoul, South Korea's emergency services trained to respond to a bomb or gas attack. An air raid siren signals a bigger attack on the population of Seoul, citizens should move underground quickly to safety. These drills are held once or twice a year and are often ignored by many. The South Korean government says there are more than 18,000 bomb shelters around the country, including subways. This one in downtown Seoul, one of many, which doubles as a bomb shelter. They've also produced this documents, brochures, telling citizens what to do in case of emergencies, including the outbreak of war. But what they couldn't tell us was where we could find these brochures and how many had actually ended up in the hands of the public. There's a distinct lack of concern on the sunny streets of Seoul, a deep-rooted sense of business as usual. For one simple reason: South Korea was being technically at war with its northern neighborhood since 1953 when an armistice, not a peace treaty was signed. The threat of attack is constant but also distant. This man says, "I'm nearly 70 years old. If I was worried, I would have emigrated. There can't be a second Korean War." As the U.S. military beefs up its presence in Korean waters, experts wonder what President Trump means when he says he'll go at it alone if he has to, and whether all options are really on the table. The national security adviser to a former South Korean president says even a preemptive strike on North Korea by the U.S. does not necessarily mean war. CHUN YUNG-WOO, KOREAN PENINSULA FUTURE FORUM: I don't believe that Kim Jong-un (ph) is crazy. I don't think he's interested in the self-destruction. So, he will be very careful. HANCOCKS: As the rhetoric increases in Washington and Pyongyang, Seoul for one is staying calm. Paula Hancocks, cnn, Seoul.