The Cheonan sank after an as yet unexplained blast on 26 March
By John Sudworth
BBC News, Seoul
Since the end of World War II only two navies, the British and the Pakistani, are known to have used a submarine to sink a warship.
Now though there appears to be growing evidence that North Korea's underwater fleet may have become the third.
The 26 March sinking of the Cheonan, with 40 lives lost and six men still missing, is certainly a South Korean military disaster.
But it has the potential to become much more than that.
If concrete proof of the North's involvement is eventually produced, it would reinforce with shocking clarity just how easily this smouldering cold-war conflict could reignite.
And it would present the international community with a serious strategic challenge - how to send a message of deterrence without risking further escalation?
The shattered wreck of the 1,200-tonne gunboat has now been winched to the surface, in two pieces, and is being examined at a naval dockyard.
The investigation team includes American, Australian, Swedish and British experts, in part, to ensure that its conclusions are seen as free from South Korean political influence.
And after an initial examination the following observations and explanations have been announced to the public.
- The skin of the ship was bent inwards, pointing to an external rather than an internal explosion, a conclusion given further weight by the fact that the ship's weapons storage area is intact
- There are no signs of scraping, or of a collision, ruling out the possibility that the ship ran aground
- There is no evidence of soot or melting on the skin of the ship, suggesting that the external explosion took place some distance away from the hull
Little wonder, then, that suspicion is mounting, with South Korean Defence Minister Kim Tae-young concluding that a torpedo attack is among the "most likely" causes.
An underwater non-contact explosion is exactly what many modern torpedoes are designed to produce, because the shock-wave from such a blast can cause much more damage than a direct hit.
And North Korean submarines, capable of carrying these kinds of torpedoes, are known to have been operating off the Korean coast at the time of the sinking.
A further clue perhaps lies in the location of the blast, close to the gas turbine room, much of which was destroyed.
"Acoustic homing" torpedoes, of the kind North Korea is thought to possess, can track and target the engine noise from a ship.
The fact that there was no warning of an attack from the Cheonan's sonar operators does not necessarily make a torpedo strike unlikely.
The South Korean Defence Ministry has been quoted as saying that in the busy, shallow waters of the Yellow Sea, a torpedo fired from a range of 2km (1.25 miles) would have a 30% chance of remaining undetected.
And there are precedents.
In 1987, for example, while on patrol in the Persian Gulf, the USS Stark was struck by two anti-ship missiles, fired from an Iraqi fighter plane, neither of which were picked up by the ship's radar.
But the torpedo theory is called into question by at least one aspect of the incident - there were no unusual military movements picked up from North Korean forces prior to the sinking.
If North Korea was planning a torpedo attack, knowing just how provocative such an action would be, would it not at least have boosted its naval defences?
The final question that should be asked is, what would North Korea have to gain from sinking a South Korean warship?
There is another explanation that could fit the scenario of an underwater, non-contact explosion and one favoured by the naval warfare expert, Norman Friedman.
"If it's a torpedo firing then that's about as big a thing as you can do short of rolling across the border," he told me.
"Unless you have a desire to start World War III then you don't do it. That's why I put my money on a mine."
Mines that were in use at the time of the Korean War were sophisticated enough to distinguish between big and small ships, and could be primed to detonate some distance from the hull.
The sinking of the Cheonan was a military disaster for South Korea
Could the Cheonan have had the misfortune to run into one that had been lying undisturbed for more than half a century?
The torpedo theory is given added weight by the circumstantial evidence - the fact that the sinking took place in disputed waters close to North Korea, where the two navies have clashed a number of times.
What is missing, at least from what we have been told so far by the investigation team, is conclusive proof - a fragment of a North Korean weapon - that would show beyond doubt what sunk the ship.
And with the strong currents surrounding the area, any evidence may have long been swept away.
Some observers have suggested that the South Korean government may prefer that evidence, if it exists, to remain undiscovered because of the political difficulty of formulating a response.
Military retaliation is highly unlikely because of the danger of escalation and because at the very least, it would panic the markets and damage the South Korean economy.
North Korea is already subject to United Nations sanctions
The diplomatic route through the United Nations is also problematic because North Korea is already one of the most isolated and sanctioned countries on the planet.
And yet, if clear evidence of an attack on the warship is produced, South Korea will of course want to send a message that such acts cannot be tolerated.
But how? North Korea is often said to have learned the lessons of the second Gulf War its own way, precisely the opposite lesson, in fact, than the one the invasion of Iraq was meant to send to so called "rogue states".
It would lose a conventional war of course, but not without first inflicting unimaginable damage on the South Korean capital, Seoul with a combination of conventional, chemical and biological artillery.
And it has a growing nuclear weapons programme, precisely it says, to guarantee its survival from a hostile outside world.
But the final question that should be asked is, what would North Korea have to gain from sinking a South Korean warship?
Using a submarine in such an attack is an act of extraordinary provocation, and one that goes a big step beyond the previous surface engagements between the two navies.
Despite North Korea's military strengths, it would still be a very risky act indeed.
If it is shown to be a torpedo that hit the Cheonan, then perhaps it can be seen as retaliation for the fact that North Korea is reported to have come off worse in the most recent naval skirmish.
Or maybe it was an attempt to rally the military around the leadership of the ailing Kim Jong-il, reportedly trying to manage a difficult transition of power to his youngest son.
But others have suggested that it might be the military acting alone, a sign of a dangerous shift in the balance of power inside North Korea, and a far more worrying prospect.