From Li Bai's (
李白) Tang poem 《
蜀道难》 (Hard is the Road to Shu):
蜀道之难,
难于上青天 — 'The road to Shu is hard, harder than climbing to the blue sky.' The poem opens and closes with this line, describing the mountain path into Sichuan. Today it's a stock hyperbole for any extreme difficulty.