a needle in a haystack idiom
不可能（或极难）找到的东西；大海里的针
something that is impossible or extremely difficult to find, especially because the area you have to search is too large
Finding the piece of paper I need in this huge pile of documents is like looking for/trying to find a needle in a haystack 要在这一大堆文件里找到我需要的那一张纸就像是（企图）大海捞针一样。