Sounds like more North Korean propaganda to try and strike fear in South Korea and impress their own domestic audience with something that probably doesn’t even work:

North Korea has tested a new nuclear-capable underwater attack drone, state media reported on Friday, as leader Kim Jong Un warned joint military drills by South Korea and the U.S. should stop.
During the test, the new North Korean drone cruised underwater at a depth of 80 to 150 metres (260-500 feet) for over 59 hours and detonated a non-nuclear payload in waters off its east coast on Thursday, North Korean state news agency KCNA said.
Reuters
You can read more at the link, but North Korea claims they can make underwater nuclear tsunamis to destroy naval fleets.








