They had to keep their discovery a secret until they had a legal claim to the land where they had found the gold.

VOA: special.2009.02.04

How to gain, how to keep, how to recover happiness, is in fact for most men at all times the secret motive of all they do, and all they are willing to endure."

如何获得，如何保持，如何重获快乐，是多数人时刻怀有的秘密动机，是他们愿意忍受一切的目的“

哈佛公开课 - 幸福课课程节选

The War Department wanted to keep the operation secret.

VOA: special.2009.07.23

The Clintons tried to keep the wedding top secret.

VOA: special.2010.08.09