大海捞针 – da hai lao zhen

dà hǎi lāo zhēn

大海捞针 (大海撈針)

try to fish a needle out of the bottom of the ocean

to find a needle in a haystack; strive to do the impossible

Variant:

hǎi dǐ lāo zhēn – 海底捞针 (海底撈針)