Sphinx's riddle字面意思斯芬克斯的谜题，引申为难解之谜。

【例句】
How the massive stones were brought here from hundreds of miles away remains a Sphinx’s riddle.
这些巨石是如何从数百英里外的地方搬到此处的，这仍然是个难解之谜。

The question was like the Sphinx's riddle to them.
这个问题对他们可谓难解之谜。