South Korean and American conspiracy theory enthusiasts have persuaded the mass media to report accusations that the South Korean warship sunk by a North Korean torpedo four months ago, was sunk by some other means. One unlikely theory is that the warship ran aground, while others suggest a rogue naval mine or a sinister plot by the South Korean government to sink its own ship and blame it on the North Koreans. These conspiracy theories are quite popular, with about 20 percent of South Koreans believing this one. Meanwhile, many people believing that gangsters (or the CIA or whatever) killed president John Kennedy, that the CIA invented AIDS and that the United States destroyed the World Trade Center with explosives on September 11, 2001.
Back in the real world, it was only two months ago that South Korea confirmed that a North Korean torpedo sank the South Korean corvette Cheonan (and killed 46 of its crew) in March. North Korea denies any culpability, but the evidence against North Korea was pretty overwhelming. Once the entire ship (which broke in two after the explosion) was recovered, it was only a matter of time before the cause of the sinking was determined. That's because a lot of very smart people, made a big effort, to reconstruct the incident.
On April 15th the aft part of the warship Cheonan was recovered, and returned to land for closer examination. The international (America, Britain, Sweden and Australia sent shipwreck experts) investigation team needed about a month to thoroughly examine the wreckage and conclude what, and who, destroyed the ship. The 74 technical experts put the Cheonan back together, just as aircraft crash investigators do, and it was pretty clear that the ship had been hit by a torpedo. That was confirmed by the discovery of torpedo fragments (like the torpedo propeller and parts of the torpedo engine, all located at the rear of the torpedo, and most likely to survive). South Korea had captured North Korean torpedoes in the last decade, and was able to match the torpedo fragments found in and around the Cheonan, with those found in disassembled North Korean torpedoes. Some of the torpedo parts found near the Cheonan also had identical Korean alphabet notations on them (compared to the captured torpedo.) The surviving South Korean sailors were also interviewed, and their recollections were consistent with a torpedo explosion.
All this evidence is either denied or explained away by conspiracy theorists. It's what they do. And on a slow news day, mass media outlets will pick up the story and run with it. It's what they do.