They do this a handful of times per year whenever the US and the South have their military exercises at sea. DPRK doesn't have a weapon capable of firing on the US mainland and their supposed largest submarine fleet in the world is a laugh riot. They have plenty of missiles stationed at strategic areas to lob at Seoul, way more than Hamas has to send into Israel. So, that's about all they got. It must creep Jr Kim out when he views footage of the South and US showing off their carrier and destroyer fleets w/ all the suped up aircraft on board. Anyways, this may be old news, it's hard to tell since it's so routine.
Yes I know its routine that's why I posted I thinks its funny.