what's that?; what the?; what's going on?
Casual interjection expressing surprise, confusion, or disbelief about something just seen, heard, or mentioned. Often used when encountering something unexpected or hard to understand.
Huh, what's that? I've never heard of it.
What's going on? What's with you all of a sudden?
Variant with katakana ソレ for emphasis or casual tone.
Full katakana spelling, often used in manga or casual messaging for emphasis.
Combination of 何 (nani, 'what') and それ (sore, 'that'). A straightforward colloquial phrase.