An escalating threat
If this latest test is confirmed as a launch from a submarine, it will require a complete reassessment of the North Korean threat, as undersea launches are far more difficult to detect than those on land.
In July, North Korean state media showed leader Kim Jong Un inspecting a newly-built submarine. The army in South Korea analyzed the images and concluded the new submarine was capable of launching three ballistic missiles.
A group of researchers in the US had noted that construction of the new submarine was nearly complete. It is possible that the latest projectile was fired from this vessel.
Missile falls inside Japan's EEZ
It is important to note that this latest missile fell into the sea inside Japan's exclusive economic zone, signaling a challenge to Japan, a US ally.
Japan's defense ministry said the missile was fired at a steep angle, known as a "lofted trajectory." One official said Japan would be within range if the projectile was launched at a normal angle.
The test also may have been aimed at ratcheting up pressure on the US ahead of the resumption of talks.
Pyongyang wants to talk with Washington, but is holding to its position that the US must first lift sanctions. It has threatened to abandon diplomatic efforts if there are no results from Washington by the end of December.
There has been no progress in talks so far this year, so Kim may be trying to increase tensions to get the US to be more flexible.
What to expect
North Korean state media are expected to announce the results of the test on Thursday morning. It's likely that Kim Jong Un attended, just as he has for all previous launches.
US president Donald Trump has said he is not concerned about short-range ballistic missile tests from the North, so we will see how he responds to this latest threat.
It will also be interesting to see whether recent changes in his administration make a difference. Last month, Trump fired his national security advisor, John Bolton, who was well known for his hard line on Pyongyang.