Skua , 11 months ago

At least as recently as 2005, diesel subs were the quieter option. There was that somewhat notorious story of the Swedish one that beat an American carrier group in a wargame because the Americans just couldn't find it. I'm sure there have been developments in the equipment and methods since then - it was 18 years ago, after all - but it's still notable enough that the Americans leased the sub from Sweden for a couple of years to practice against it.

That said, the Swedish sub in question was packed with cutting-edge (at the time) stealth features. I suspect North Korea's fleet is not.