In this May 21, 1956, file photo, the stem of a hydrogen bomb, the first such nuclear device dropped from a U.S. aircraft, moves upward through a heavy cloud and comes through the top of the cloud, after the bomb was detonated over Namu Island in the Bikini Atoll, Marshall Islands. North Korean Foreign Minister Ri Yong Ho said his country may conduct a "historic" hydrogen bomb test in the Pacific Ocean. Many experts think North Korea wouldn't do something so risky, but it's hard to rule out given North Korea's steadily expanding nuclear and missile tests. AP Parts...