On a chilly spring evening late last March, a South Korean naval ship called the Cheonan was conducting routine exercises in waters just off the coast of a sparsely populated island in what the Koreans call the West Sea (better known as the Yellow Sea), just 10 kilometers from North Korean land. It was just before 9:30, and for most of the ship's 104 crew members, work was done for the day. Some sat in the ship's mess chatting; others were exercising in a small gym. A few had already headed to their bunks for a night's rest. The ship's commanding officer, Choi Won Il, had retired to his cabin for the night and was checking email.