which one is correct

[Anonymous]

Something had happened there, I don't know what?
Something had happen there, I don't know what?

[blacknomi]

Something had happened there, I don't know what it was.

Perfect tense
have/had + present participle

[Francois]

Isn't the past perfect "dangling" here?

FRC

[shane]

Isn't the past perfect "dangling" here?

I hope it doesn't drop. ;)

[Casiopea]

Something had happened there, I don't know what?
Something had happen there, I don't know what?

Comma splice
Something had happened here (Sentence 1),
I don't know what (Sentence 2).

Try,

A. Something had happened here; I don't know what (it was).
B. Something had happened here. I don't know what (it was).
C. Something had happened here and I don't know what (it was).

[Francois]

Isn't the past perfect "dangling" here?

I hope it doesn't drop. ;)

I guess this is FRC-speak
I meant can the past perfect be used when it is not tied to a precise event in the past (usually marked by a simple past clause) ?

FRC