Structure:

到底+question phrase

Meaning:

到底 can be used to mean “on earth”, where it is used to emphasize a question. In this sense, 到底 is similar to 究竟.

Examples:

1. 内奸 到底 是 谁？

nèijiān dàodǐ shì shuí？

Who on earth is the mole?

2. 到底 怎么 走？

dàodǐ zěnme zǒu？

How on earth do we get there?

3. 你最低价 到底 是 多少？我 不 想 跟 你 讲价!

nǐzuìdījià dàodǐ shì duōshǎo？wǒ bú xiǎnɡ ɡēn nǐ jiǎnɡjià!

Come on, give me your lowest price, I’m not in the mood for haggling.

4. 你 到底 怎么 了？

nǐ dàodǐ zěnme le？

What on earth is up with you?