When someone asks you "what on earth are you doing?", "on earth" is used by the speaker to intensify the question. 究竟 (jiūjìng) can be used in the same way.

Basic Structure

究竟+question

Examples

nǐ jiūjìnɡ xǐ bú xǐhuɑn wǒ?
你 究竟 喜 不 喜欢 我?
So, do you like me or not?

jiūjìnɡ chī shénme?měitiān dōu dé xiǎnɡ zhèɡe wèntí!
究竟 吃 什么?每天 都 得 想 这个 问题!
What on earth do you want to eat? We have this problem every day!

zhècì ōuzhōubēi de ɡuànjūnjiūjìnɡ shì nǎɡè duì?ɡuǎnɡɡào zhīhòu jiù huì zhīdào!
这次 欧洲杯 的 冠军究竟 是 哪个 队?广告 之后 就 会 知道!
Which team won the European Cup in the end? Find out after these commercials!

nǐ jiūjìnɡ kàn dào wǒ le méiyǒu?
你 究竟 看 到 我 了 没有?
Did you see me or not?!

Note that 究竟 is more formal than 到底.