Military officials and experts believe that if a North Korean torpedo was involved in the sinking of the South Korean Navy corvette Cheonan, it was probably launched from a 325-ton Shark-class submarine. The sub ranks between a full-blown submarine and a mini-sub in terms of size.

South Korean military intelligence are reportedly focusing on the fact that one or two Shark-class submarines from a submarine base in Cape Bipagot, South Hwanghae Province are unaccounted for during the time of the Cheonan's sinking.

The Bipagot submarine base is around 80 km from Baeknyeong Island. Shark-class submarines can travel at speeds of 13 km/h under water, so it would take them between six and seven hours to reach Baeknyeong Island. Intelligence officers and experts believe the sub made the trip under water, since traveling on the surface of the water would have exposed it to South Korean and U.S. spy planes and surveillance satellites.

A drawback of the Shark-class diesel-powered submarines is that they need to surface regularly to recharge their batteries and ventilate, a process known as "snorkeling." During this process, the ventilation device can be detected by radar and other surveillance equipment. "The snorkeling equipment is not big, so there is a slim chance that it was detected by South Korean radars while the sub was in North Korean waters," said one source.

The military believes a North Korean sub could have approached by taking a detour through open seas left of Baeknyeong Island, instead of coming in straight between Hwanghae Province and Baeknyeong Island. That is because the underwater currents are extremely fast in that area and it is closely monitored by South Korean forces. There is also the possibility that a sub could have drifted into waters near Baeknyeong Island with its engines shut off.

"Between 3 p.m. and 9 p.m. on March 26, the day the Cheonan sank, the currents flowed north to south and shifted direction from south to north after 9:40 p.m.," said military expert Kim Byung-ki. "There is the possibility that a North Korean sub was lying in wait and used the northerly current to return to North Korea after the attack."

Once it infiltrated waters near Baeknyeong Island, it could have fired a torpedo around 1 km to 2 km away from the Cheonan in deeper waters. And a mid-sized torpedo, weighing more than 200 kg rather than a small one weighing between 50 kg to 80 kg, is being cited as the probable weapon, judging from the huge damage the Cheonan suffered.

A lingering question is why the Cheonan's radar system was unable to detect a torpedo attack, if that was i ndeed the cause of the sinking. The Defense Ministry says the sonar aboard a South Korean warship like the Cheonan has a 70-percent chance of detecting submarines or semi-submersibles around a 2 km radius. But retired naval commanders say the chances are actually only 50 percent, so sonar officers could have been unaware of an approaching torpedo.