Earlier quoted context omitted.
SOSUS may have been classified but you could read about it 40+ years ago.
But not it's true purpose: tracking Soviet submarines.
The exact way they do it now, where they do it, etc. is the secret. The mere existence of a sensitive listening system is not a secret that endangers national security if known; the opposite is true, we want people to know/think they can't move a tennis ball underwater without us hearing it. Knowing enough about it to get past it is such a secret.