巨大的石头如何从数百英里外运到这里，仍然是个谜。

How the massive stones were brought here from hundreds of miles away remains a ___mystery.

他从未放弃解决儿子失踪之谜的希望。

He never gave up hope that he would solve the ___mystery of his son's disappearance.

他失业了，所以他如何支付租金对我来说是个谜。

He's out of work, so how he pays his rent is a ___mystery to me.