这些巨石是如何被运到这里的，仍然是一个谜。

How the massive stones were moved here remains a ___mystery.

他想解开儿子失踪的谜团。

He wanted to solve the ___mystery of his son's disappearance.

他是如何在没工作的情况下过日子的，对我来说完全是个谜。

How he manages without a job is a complete ___mystery to me.