a needle in a haystack
US /ə ˈniːdl ɪn ə ˈheɪstæk/
UK /ə ˈniːdl ɪn ə ˈheɪstæk/
1.
something that is extremely difficult or impossible to find, usually because it is very small or hidden among many other things
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Finding my old childhood toy in this attic full of junk is like looking for a needle in a haystack.
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Trying to find a specific piece of information in that massive database without proper filters is like looking for a needle in a haystack.