This is very slangy. The proper form is "Who would have thought it?" but it deliberately follows a wrong pattern of grammar (think-thank-thunk, as in drink-drank-drunk).
It's often said jokingly, perhaps to indicate that the speaker is so surprised by the news that all sense of grammar has been lost, or that the speaker imagines him/herself to be stupid enough to actually say "thunk". A case of woefully wrong grammar used for effect.
The other form is "Who woulda thunk it?"