> And why couldn't the Navy tell they heard it implode?

The microphone field is intended to listen for Russian submarines: had the information been released immediately, it's possible someone might have been able to work out where the microphones are. My guess is that it took time enough for someone to work out that merely revealing that this capability exists didn't compromise US security, plus time for any potentially-compromising information to decay to uselessness (note also how *little* information they released), plus declassification paperwork propagation delay.

We're lucky we heard about it so soon. It took years before the US military declassified their observations of astronomical gamma-ray bursts: the satellites which detected the phenomenon were intended to look for nuclear detonations. Declassification might well have had to wait until said satellites were superseded.