like looking for a needle in a haystack字面意思如同在稻草堆里找一根针，引申为大海捞针。

【例句】
Searching for one man in this city is like looking for a needle in a haystack.
在这个城市里找一个人无异于大海捞针。

Gosh! It's just like looking for a needle in a haystack in a library with many thousands of books.
天啊！在有数十万册书的图书馆内找，就好像大海捞针一样。