Between 70 and 80 percent of North Korea's submarine fleet is stationed along the eastern coast, where four shark-class submarines disappeared recently from South Korean radars. Compared to the shallow waters of the West Sea, conditions in the East Sea are so favorable to submarines that it has been referred to as a "paradise" for them.
North Korea has around 70 submarines -- 20 Romeo-class subs weighing 1,800 tons, 40 shark-class subs (325 tons) and 10 salmon-class subs (130 tons). A salmon-class sub is believed to be responsible for sinking the South Korean Navy corvette Cheonan.
There are four North Korean submarine bases along the east coast, including Chaho Base where the four shark-class subs that vanished are stationed, as well as Mayangdo, Toejo and Wonsan, all in South Hamgyong Province.
Chaho and Mayangdo are the main bases. Chaho is equipped with a cave to protect submarines from aerial attacks as well as a canal that can transport submarines faster to the ocean. A Google Earth image unveiled a few years ago shows eight Romeo-class and three shark-class submarines at Chaho.
The Mayangdo Base is near the site of an abortive light-water reactor project in Sinpo and is ideal for safe docking and hiding submarines. It apparently houses Romeo, shark and even whiskey-class training submarines. The base in Toejo is home to North Korea's eastern naval command and the shark-class submarine that was stranded off the coast of Gangneung in 1996 carrying 25 North Korean spies.
Using the East Sea, which makes it difficult to detect submarines, small North Korean submarines apparently infiltrated South Korean waters regularly during the 1990s. A log found aboard a yugo-class North Korean submarine captured off the coast of Sokcho in 1998 showed records of numerous infiltrations. South Korea's First Naval Command, which covers the East Sea, has dispatched destroyers, convoys and corvettes to search for the four submarines that have disappeared from radars.