Originally posted by Ponderable The adjectives "deep" and "dark" hint that the information in question is existential (deep) and not positive (dark).
I agree about "dark" but I think that "deep" can just be an intensifier meaning "very".
So if someone has a deep secret, then you are unlikely to drag it out of them, or perhaps you have no idea that there is a secret; perhaps not only is there a secret but the fact that there is a secret is also secret; that would be "deep".
So, for example, everybody knows that something dark happened when I was in Jakarta but what it was is secret. If nobody knows that the dark thing happened because I have not even volunteered that much information, then maybe we could say it's a deep, dark secret.
For me the deepest and darkest secrets are the kind of secrets people keep to prevent exposing their true personality.
Who they're preventing this exposure from is the real question... how many people have been married with kids for decades and held on a certain secrets in the fear of changing the dimensions of said relationships.