as secret as the grave

例句：

1. 1.The secret of his success was kept from everyone.
   他成功的秘密被大家保密了。
2. 2. The treasure was so well-concealed that only a few knew its location.
   宝藏藏得如此隐秘，只有少数人知道它的所在。
3. 3.The couple's love story is as secret as the grave.
   这对情侣的爱情故事就像坟墓一样不为人知。