大海捞针----look for a needle in a haystack

例句：
A:Could yon please find these materials in the library for me?
请你帮我在图书馆内找出这些资料好吗？
B: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寻找，那不像大海捞针一样吗！