This so-called "show of force" comes during the same month in which North Korea conducted its fifth nuclear test just days after successfully launching three missiles into the Sea of Japan.
The North, for its part, claims it tested a nuclear warhead -- which can be placed on top of a missile -- though there is no way to verify that claim.
And the possibility of pairing a nuclear warhead with a missile is all the more frightening when you consider the country has also been testing how to launch missiles underwater, where they're harder to detect.
"The question that some experts are raising is whether or not the North Koreans can actually mate a miniaturized nuclear warhead onto such a missile," Alexander Neill, a North Korea expert at the International Institute for Strategic Studies Asia," told CNN.
"If there's evidence that they can do that -- or they have done that -- than this is major concern for the region."
Those tests come just after North Korea had what many experts believe to be its first successful submarine missile launch in August.
"While this was a substantial improvement in North Korea's demonstrated capabilities, it does not likely represent an operational submarine launched ballistic missile capability at this time," John Schilling, an aerospace engineer and contributor to the North Korea monitoring project 38 North, told CNN in an email after the sub missile launch.
The missile traveled 311 miles (500 kilometers) -- and was the first projectile ever fired by the North Koreans to reach Japan's air defense identification zone, according to Japanese Prime Minister Shinzo Abe.
But even the possibility of an underwater missile launch poses a new set of problems for North Korea's neighbors -- it's a wild-card factor that introduces "a new, complex dimension of threat for the ROK [Republic of Korea], the US, Japan and others ... the ability to keep an adversary guessing, or at least to absorb an adversary's resources in tackling a new threat," says Neill.
"This new capability will demand a response from the US and its allies," he said. "It has introduced a new dynamic into the threat matrix on the Korean peninsula."
The one sub
Based on pictures that were released by North Korea, the submarine is believed to be its more modern, Gorae-class sub -- North Korea reportedly only has one -- according to an analysis by IHS Jane's.
It's also likely the only sub they have that can fire a ballistic missile.
The Gorae submarine is largely shrouded in mystery -- it's not clear if the North Koreans are planning to use it as an experimental vehicle or whether it will be replicated and reproduced, Jane's says.
And the test itself was an audacious and risky move, Schilling says.
"Testing from a submarine shows great confidence from the North Koreans, almost recklessly so," he said.
"The solid-fuel KN-11 is basically a new design, and North Korean missiles almost never work right on their first try. They took a big risk of damaging or sinking their only ballistic missile submarine, something we wouldn't have expected this soon, and it paid off for them (this time). "
The rest of North Korea's fleet is mostly older, Soviet-era submarine equipment.
The country has about 70 submarines in its fleet, according to various independent estimates.
And those subs are louder and easier to detect, according to Neill.
Some of them are older, Soviet-era pieces of equipment, while others were transferred from China in the 1970s.
The missile fired from a sub appeared to be a solid fuel KN-11 -- "basically a new design," Schilling said.
The missile is typically nine meters (30 feet) long. It's not clear what its range is.
Though the North Koreans are getting closer, most experts believe they are still a ways away from having a viable submarine-based missile launch system.
"We would expect the first deployment of an operational system to occur in about two years, with full capability involving multiple submarines a year or so after that," Schilling said.
"They might be able to put to sea with the one experimental submarine they presently have, some time next year, but that would be a risky move that would give only a very limited and unreliable capability."