N3
一体
いったい
(What) on earth?, (Who) in the world?, (Where) on earth?
一体ここで何が起きたんだ！
What the heck happened here?
Explanation
「一体」 (いったい) goes before a question word (何, 誰, どこ, どうして) and strengthens the question: 'what on earth', 'who in the world'. Faced with something strange, chaotic or unbelievable, the speaker is puzzled, anxious, even angry: 「いったい何が起こったんだ」 (What on earth happened?).
Mixed up with
It is strong. Asking a stranger or an elder 「いったい何ですか」 sounds aggressive and rude. In formal business, soften it, e.g. 「どのようなご事情でしょうか」.
In essays, authors often open a deep reflection with a question like 「人類とは一体何なのだろうか」.