Hay

“While it is possible to find a needle in a haystack, even using specialized machines to do so takes a considerable amount of time, particularly since bone needles cannot be picked up by magnets. The task is difficult enough to still make the saying viable.” – MythBusters, Needle in a Haystack Episode

And that’s with knowing 4 needles existed somewhere within 10 bales of hay. How difficult would it be if you didn’t know the needles existed – just assumed they did.