look for a needle in a haystack 大海捞针
例句：
To find her in such a big city is like looking for a needle in a haystack.
在这么大的城市里找到她就像大海捞针。
Finding a hotel in the holiday season here is like looking for a needle in a haystack.
在度假旺季的时候，要想在这里找到一家旅店简直是大海捞针。
A:We are supposed to meet John here at the train station.
我们要在火车站接约翰。
B:That's like looking for a needle in a haystack.
那简直是大海捞针。