This article by investigative reporter Tanaka Sakai points to the possibility of friendly fire between two vessels.
"But around the time of this incident another sinking occurred that has hardly been reported in Japan. Near the site of the sinking of the Cheonan, a colossal object, which appears to be a US submarine, was found to have sunk. An ROK underwater team searched for, and on April 7 South Korea’s KBS TV showed, a US helicopter carrying what seems to be the body of a US soldier. KBS is a public broadcasting station with the highest credibility in South Korea."
Since this report was published, US Submarine Columbia has returned to Hawaii.
"ROK and US authorities did their best to hide the fact that a US submarine sank at about the same time as the Cheonan.... On the day of the incident, the exercise was underway. After the incident, the US-ROK authorities made no mention of the fact that the joint military exercise was in progress. But the day after the incident, various ROK media and newspapers reported that the Cheonan might have been sunk by friendly fire during the military exercise."
On 26 March, 2010 near Baengnyeong Island (White Wing, also known as Baekreong) to the South of the northern limit line, the maritime demarcation line between South and North Korea, South Korea’s large patrol boat Cheonan (Heaven’s Peace) exploded and sank. Already, more than one month after the accident, the cause of the sinking has not been confirmed. In early April, the South Korean government announced that either a torpedo struck or an underwater mine exploded, sinking the ship, indicating that it was not destroyed by an explosion or accident inside the boat but by an external cause.
The stern of the Cheonan docked on a barge off Baengnyeong Island on 7 May, 2010. Lee Jung-hoon.
Map of Baengnyeong Island (1)
Map of Baengnyeong Island (2)
A map provided by KBS TV. The third buoy to the East of Baengnyeong Island is where the head of the Cheonan sank, and the rear of the Cheonan sank to the West.
The map of the search generally reported: two black dots to the South of Baengnyong are where the halves of Cheonan reportedly sank. The third buoy is not shown.