South Korea and the U.S. were reportedly engaged in a joint antisubmarine drill at the time of the sinking of the "Cheonan" warship on March 26th in the Yellow Sea.

The Associated Press cited U.S. Forces Korea spokeswoman Jane Crichton as saying the military exercise began at 10 p.m. on March 25th and ended at 9 p.m. the next day.
The night before the Cheonan sank, two U.S. destroyers and other ships practiced tracking and maneuvers while a South Korean submarine played the role of target some 120 kilometers from where the warship sank. The exercise was terminated because of the blast aboard the Cheonan.

The submarine drill was part of the annual Key Resolve and Foal Eagle exercises between the U.S. and South Korea.

A Navy official in Seoul confirmed that the two countries were training in waters west of Taean Peninsula in South Chungcheong Province at the time of the Cheonan's sinking. He did not confirm whether it was an antisub drill.