헐 ~ an exclamation that is made when surprised or mildly shocked, a bit like what?/really?/seriously?/no way!
Example: I shared a bit of history of the royal family (from a couple of hundred years ago I should say) with my language partner. The conversation turned to one queen in particular and went roughly like this:
-여왕은 왕의 의사와 불륜 관계였어요 ~ the queen had an affair with the king’s doctor.
-헐! ~ what? No way!
Note that you will never find 헐 in a formal setting