finding a needle in a haystack字面意思：寻找落进干草垛里的一根针 ，引申为“大海捞针”。

【例句】
The thief ran quickly and disappeared in a sea of faces in the subway, leaving the policeman with a job like finding a needle in a haystack.
小偷跑得很快，消失在地铁的人山人海里。警察要找他好像大海捞针一样。

Trying to get a new job these days is like trying to find a needle in a haystack.
这些天想找一份新工作就像大海捞针。