Needle In a Heystack - 大海捞针

大(da)海(hai)捞(lao)针(zhen)

"大海" means "the sea"
"捞" means "look for , search for"
"针" means "needle"

It means it's so difficult to find something or somebody, like dredge for a needle in the sea, accurately termed "needle in a heystack" in English .