The Japanese apparently had a nuclear weapons research program in WW2 in part thanks to Nazi Germany but they would have tested any weapon... and a Weapons test would have been noticed. So when does a nuclear detonation go unnoticed? When we are sure its an Earthquake.
Mikawa Bay was the site of a terrible quake January 13, 1945 and was apparently eleven miles deep. A thousand dead and a thousand missing.
That can't possibly be a nuclear test.
But a subterranean nuclear test could have triggered the larger quake simply because it was on a major fault line. A sufficiently large enough subterranean test would have sent out a shockwave.
Of course there would need to be evidence. Like say a circular subsidence event on the bay floor. Well oddly enough...there is. And its big. Very very very big. Kilometres in diameter big. Given the subterranean test of a twenty five kiloton device produced a crater as big as a carpark, Megatons big. A coal mine below the bay would have dropped the overlying strata and absorbed betaparticles.
Perhaps most telling: this was the only quake during WW2 in which the Japanese Government censored reports of it.
And there is Mikawa bay with a circular structure near three islands. Sure its a poor quality image but it gives us sufficient evidence to investigate further.