Unusual submarine activity by North Korea may be a harbinger of submarine-launched missile tests to come, a worrying development given the outlaw regime’s recent advances in intercontinental ballistic missile technology.

On Tuesday, CNN quoted a defense official who said the American military has detected “highly unusual and unprecedented levels” of North Korean sub activity, along with evidence that a cold-launch system for long-range missiles has been tested. The cold-launch system is, in essence, a steam cannon that blows a missile high enough into the air for its engines to ignite without damaging the submarine or interacting with seawater.

The ejection tests described by CNN’s source occurred on land at North Korea’s Sinpo Naval Shipyard. If these tests were deemed successful, presumably the next step would involve test-launches from deployed submarines, which makes the news that at least three North Korean subs have been detected operating off the coasts of Korea and Japan disturbing. One of the subs near Japan has reportedly sailed further than ever before and is not behaving in a manner consistent with training activity.

“Land-based and submarine-based missiles are considered two-thirds of what is known as the ‘Strategic Triad,’ a theory that a state must have land, air, and sea-based nuclear attack capabilities to successfully deter an enemy from trying to attack it,” CNN explains.

North Korea’s submarine launch program is believed to remain in its infancy, with most of its 70-odd submersible vehicles incapable of carrying missiles. Then again, Pyongyang’s ICBM program just turned out to be far more advanced than analysts expected.

The North Korea-watchers at 38North suspect something unpleasant is coming, based on satellite photographs of activity at the Sinpo shipyard and another facility at Nampo. Barges that appear similar to those used in old Russian submarine missile tests have been spotted at both locations, suggesting a significant acceleration in North Korea’s sub-launched missile program.

South Korea is taking the situation very seriously, with military analysts warning that more missile tests and another nuclear test could be right around the corner. The Korea Herald reports that South Korean defense officials are considering their own nuclear submarine program to counter the North Korean sub threat, along with the development of more powerful land-based missiles.

In another development under-reported by Western media, the Korea Herald notes that North Korea’s last ICBM watch appears to have eliminated resistance by President Moon Jae-in’s administration to deploying the American THAAD anti-missile shield in South Korea. The United States announced successful results from a new THAAD test on Sunday.

South Korean media on Tuesday described a plan by South Korea’s Joint Chiefs of Staff and Ministry of National Defense to conduct a massive “surgical strike” to destroy North Korea’s missile and nuclear facilities if necessary.

One bit of good news amid the rising alarm on the Korean peninsula: U.S. and South Korean analysts believe North Korea’s last ICBM test did not include the shielding necessary to allow a nuclear warhead to survive atmospheric re-entry. Based on footage captured by Japanese cameras, the analysts believe North Korea’s test missile disintegrated before it struck the ocean. However, it appears North Korean scientists might be only six months, and a few more missile tests, away from developing the shielding they require.