Baengnyeong Island sits less than ten miles from the North Korean coast and over a hundred miles from the South Korean mainland, a geographic fact that makes everything about it contested. The island lies south and west of the Northern Limit Line, the de facto maritime boundary drawn unilaterally by United Nations Command in 1953 at the close of the Korean War. North Korea has never recognized the line. The surrounding waters hold rich fishing grounds used by North Korean and Chinese vessels, and both Koreas claim them as territorial waters. Naval clashes in the area have occurred repeatedly -- small engagements sometimes called "crab wars" that rarely make international headlines but never fully stop. The night of the sinking, the US and South Korean navies were conducting joint anti-submarine warfare exercises 75 miles away. Into this volatile maritime no-man's land, a 1,200-ton corvette was on routine patrol.

A multinational investigation team -- including experts from South Korea, the United States, the United Kingdom, Canada, Australia, and Sweden -- concluded that the Cheonan was sunk by a North Korean torpedo. The specific weapon was identified as a CHT-02D, a North Korean-manufactured torpedo using acoustic and wake-homing guidance. Investigators recovered torpedo fragments from the seabed, including propeller blades, a propulsion motor, and a steering section that matched schematics from North Korean export brochures. Traces of RDX, a high explosive used in torpedoes, were found on the hull. A marking in Korean script reading "No. 1" was discovered on the propulsion section. The team concluded that the torpedo detonated several meters below the hull without contact, creating a bubble jet and shockwave that split the ship in two. But the conclusion was not unanimous beyond the official team.

Russian Navy submarine and torpedo experts visited South Korea, examined evidence, and took samples home for analysis. No official Russian report was ever published, but leaks to the press suggested their team was not convinced by the torpedo theory. According to the South Korean newspaper The Hankyoreh, the Russian investigators concluded that the Cheonan had touched the sea floor and damaged a propeller before a non-contact explosion -- possibly from a mine rather than a torpedo -- caused the hull to break. They reportedly found that the recovered torpedo parts appeared to have been in the water for more than six months, an observation difficult to reconcile with a weapon fired that night. Chinese officials reportedly told American counterparts that the sinking was caused by an American rising mine planted during the joint exercises, an explanation the US dismissed. A separate investigation by scientists at the University of Manitoba found that residue identified as aluminum oxide -- a torpedo byproduct -- had an oxygen-to-aluminum ratio inconsistent with an explosion. In South Korea itself, surveys showed that less than a third of the public fully trusted the official findings, though a later poll put the figure at 68 percent.

Whatever the cause, the consequences were concrete. South Korea suspended nearly all trade with the North, an embargo estimated to cost Pyongyang roughly $200 million annually. The US and South Korea announced joint naval exercises in response. North Korea denied involvement, warned of "ruthless punishment" if held accountable, cut most remaining ties and communications with the South, severed the naval non-aggression agreement, and announced that any South Korean vessel crossing the disputed maritime border would be attacked. A UN Security Council presidential statement condemned the attack without naming the attacker -- a diplomatic compromise reflecting China's resistance to a harder line against Pyongyang. Inside South Korea, the government pursued critics who questioned the investigation, charging a former presidential secretary with defamation and announcing crackdowns on "groundless rumors" spread online. A South Korean military oversight board later accused senior naval leaders of lying and hiding information about the incident. The Cheonan itself was raised, examined, and preserved. In December 2012, a North Korean defector claimed that the submarine crew responsible had been honored as "Heroes of the DPRK" in October 2010. Forty-six sailors remain at the Daejeon National Cemetery.