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North Korea says it has successfully carried out its fifth nuclear test. The announcement on state media came hours after a seismic event was detected near its nuclear test site. South Korea believes it is the North's biggest-ever test, raising fears it has made significant nuclear advances. So how concerned should we be about this latest test? Jean H. Lee is a North Korea watcher from Yonsei University in the South Korean capital, Seoul. (Photo: A mock North Korean missile at a rally denouncing North Korea's nuclear programme, in Seoul. Credit: Kim Jae-Hwan/Getty)