cáng lóng wò hǔ

藏龙卧虎 (藏龍臥虎)

hidden dragons and crouching tigers

Undiscovered persons of exceptional ability; talented individuals in hiding; concealed talent

Example:

藏龙卧虎之地
cánglóngwòhǔ zhī dì
a place where people of unusual ability are to be found.