", although the first one sounds more colloquial to me. " when you want to ask about a continued event which is still taking place at the moment of speech. " can be used equally with the above ones.
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RuslanaI guess there's no big difference between "What's up?" and "What happened?", although the first one sounds more colloquial to me.
Use "What's going on?" when you want to ask about a continued event which is still taking place at the moment of speech.
I am not sure whether "What is it?" can be used equally with the above ones.