In March 2010 the corvette ROKS Cheonan, operating in the Yellow Sea near the Northern Limit Line, was struck in the stern by an undetected torpedo. The 1,500-ton Cheonan, a Pohang-class general-purpose corvette, broke into two halves and sank. Forty-six South Korean sailors were killed and fifty-six were wounded. An international commision set up to investigate the incident pinned the blame on North Korea, in large part due to the remains of a North Korean CHT-02D heavyweight acoustic wake-homing torpedo found at the location of the sinking. The submarine responsible is thought to have been a Yono-class midget sub.

North Korea should by all rights be a naval power. A country sitting on a peninsula, Korea has a long naval tradition, despite being a “shrimp” between the two “whales” of China and Japan. However, the partitioning of Korea into two countries in 1945 and the stated goal of unification —by force if necessary—lent the country to building up a large army, and reserving the navy for interdiction and special operations roles. Now, in the twenty-first century, the country’s navy is set to be the sea arm of a substantial nuclear deterrent.

The Korean People’s Navy (KPN) is believed to have approximately sixty thousand men under arms—less than one-twentieth that of the Korean People’s Army (KPA) ground forces. This, as well as comparable budget makes the KPN’s auxiliary role to the KPA. KPN draftees spend an average of five to ten years, so while Pyongyang’s sailors may not have the latest equipment, they do end up knowing their jobs quite well.

A substantial number of these sailors serve in the KPN’s submarine fleet, which is one of the world’s largest. In 2001, North Korea analyst Joseph Bermudez estimated that the KPN operated between fifty-two and sixty-seven diesel electric submarines. These consisted of four Whiskey-class submarines supplied by the Soviet Union and up to seventy-seven Romeo-class submarines provided by China. Seven Romeos were delivered assembled, while the rest were delivered in kit form. Each Romeo displaced 1,830 tons submerged, had a top speed of thirteen knots and was operated by a crew of fifty-four. The Romeo submarines were armed with eight standard-diameter 533-millimeter torpedo tubes, two facing aft. North Korean leader Kim Jong-un was filmed touring and taking a short voyage on a Romeo-class submarine in 2014.

Despite such an endorsement, the submarines are generally considered obsolete and are being phased out. In 2015, the Pentagon believed that North Korea has seventy submarines of unknown types on active duty. A multinational report on the sinking of the South Korean corvette ROKS Cheonan states that the KPN operated twenty Romeo-class submarines, forty Sang-O (“Shark”) class coastal submarines (SSCs), and ten midget submarines of the Yono class.

The Sang-O class of coastal submarines is approximately 111 feet long and twelve feet wide, and displaces 275 tons. It can do 7.2 knots surfaced and 8.8 knots submerged. There are two versions, one with torpedo tubes and another where the torpedo tubes are replaced with lockout chambers for divers. The latter are maintained by the KPN but operated by the Reconnaissance Bureau’s Maritime Department. An improved version, informally known as the Sang-O II, is 131 feet long, displaces between 350 and 400 tons, and reportedly has a top speed of thirteen knots. The armed variant is believed to be capable of carrying, in addition to torpedoes, sea mines, while the Reconnaissance Bureau’s version carries between thirty-five and forty passengers and crew.

Finally, North Korea has about ten Yono-class midget submarines (SSm). Derived from an Iranian design, the Yono class displaces 130 tons submerged, with two 533-millimeter torpedo tubes and a crew of approximately twenty. The submarine can make an estimated eleven knots on the surface, but only four knots submerged.

North Korea’s submarine fleet, while smaller and less well funded than the other armed services, has generated an outsized number of international incidents. On September 18, 1996, a Sang-O SSC operated by the Reconnaissance Bureau ran aground near Gangneung, South Korea. The submarine, which had set a three-man party of commandos ashore two days before to reconnoiter a South Korean naval base, had failed to pick up the party the the previous night. On its second attempt, the submarine ran aground and became hopelessly stuck within sight of the shoreline.

Aboard the submarine were twenty-one crew and and the director and vice director of the Maritime Department. South Korean airborne and special-forces troops embarked on a forty-nine-day manhunt that saw all of the North Koreans except for one killed or captured. Many committed suicide or were murdered by their superior officers to prevent capture. The remaining North Korean sailor, or agent, is believed to have made his way back across the DMZ. Eight ROK troops were killed, as were four South Korean civilians.

In 1998, a Yugo-class midget submarine, predecessor to the Yono class, was snared in the nets of a South Korean fishing boat and towed back to a naval base. Inside was a macabre sight: five submarine crewmen and four Reconnaissance Bureau agents, all dead of gunshot wounds. The crew had been murdered by the agents, who promptly committed suicide. The submarine was thought to have become entangled in the fishing boat’s net on its way back home to North Korea, after picking up a party of agents who had completed a mission ashore.

In March 2010 the corvette ROKS Cheonan, operating in the Yellow Sea near the Northern Limit Line, was struck in the stern by an undetected torpedo. The 1,500-ton Cheonan, a Pohang-class general-purpose corvette, broke into two halves and sank. Forty-six South Korean sailors were killed and fifty-six were wounded. An international commision set up to investigate the incident pinned the blame on North Korea, in large part due to the remains of a North Korean CHT-02D heavyweight acoustic wake-homing torpedo found at the location of the sinking. The submarine responsible is thought to have been a Yono-class midget sub.

North Korea’s latest submarine is a step in a different direction, the so-called Sinpo or Gorae (“Whale”) class ballistic-missile submarine (SSB). The SSB appears to blend submarine know-how from previous classes with launch technology from the Soviet Cold War–era Golf-class ballistic-missile submarines; North Korea imported several Golf-class subs in the 1990s, ostensibly for scrapping purposes. Both the Golf and Gorae classes feature missile tubes in the submarine’s sail. The tubes are believed to be meant for the Pukkuksong-1 (“Polaris”) submarine-launched ballistic missiles currently under development. If successful, a small force of Gorae subs could provide a crude but effective second-strike capability, giving the regime the opportunity to retaliate even in the face of a massive preemptive attack.

North Korea’s reliance on submarines exposes a harsh reality for the country: U.S. and South Korean naval and air forces are now so overwhelmingly superior that the only viable way for Pyongyang’s navy to survive is to go underwater. While minimally capable versus the submarine fleets of other countries, North Korea does get a great deal of use out of them. Although old and obsolete, North Korea’s submarines have the advantage of numbers and, in peacetime, surprise. Pyongyang’s history of armed provocations means the world hasn’t seen the last of her submarine force.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009, he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.

This first appeared in April.

Image: Reuters.