海底捞月. [hǎi dǐ lāo yuè]

• try to fish out the moon from the bottom of the sea -- strive for the impossible [illusory]; a fruitless attempt; a useless effort; cry for the moon; dredge for the moon in the sea; fishing for the moon in the sea:

  be as futile as fishing for the moon in the sea

  海底捞月一场空