The Poseidon dual-core unmanned underwater vehicle (nuclear-powered, nuclear-warhead torpedo), named “Tsunami” by North Korea, was tested by North Korea from March 21 to 23, and the submersible submersible submerged 80-150 meters underwater to the vicinity of the Sea of Japan, hidden for more than 60 hours and then exploded by remote control, which was not found by the United States, Japan and South Korea before the explosion.