Trouble on the Korean peninsula? A South Korean naval ship sank on Friday after an explosion near the maritime border with North Korea. A rescue mission is under way to save the ship’s 100 or so sailors, 40 of whom are reported missing. Shortly after the explosion, a South Korean ship fired shots at an unidentified vessel; if it was North Korean, it would be the second time in five months that the Koreas have exchanged fire at sea. Korean TV reports have speculated that the ship was sunk by a torpedo, but so far there is no official word.