North Korea has lost contact with one of its submarines, U.S. officials tell CNN. The submarine, apparently on some sort of exercise earlier this week, was being tracked by the U.S. military when it suddenly stopped. The fate of the sub is unknown, but U.S. forces have observed North Korea's attempts to find the missing craft. U.S. officials believe the submarine experienced some sort of failure, and the U.S. military has been monitoring the subsequent search and rescue mission via spy satellites, aircraft, and ships. The incident is taking place at a time of particular high tension in the area,...