What goes on in a cocoon as a caterpillar changes into a butterfly remains one of nature's best kept secrets.

当毛毛虫变成蝴蝶的时候，茧里发生了什么仍然是自然界最大的秘密之一。

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In the west, it's one of the best kept secrets.

在西方，这是保守得最好的秘密之一。

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I think it was really one of the best kept secrets ever.

我认为这确实是保守得最好的秘密之一。

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One of the best kept secrets about the EU is that it has already done that in a number of ways.

BBC: Europeans on Europe

One of the best kept secrets of the Midwest is that Kansas City, Missouri is home to a thriving entrepreneurial community.

FORBES: Kansas City Is Boosting Its Tech Startups With Launch KC

"This airport is one of the best kept secrets around, " he said.

WSJ: Pitching Duel: In L.A., Fight