From Bai Juyi's (
白居易, Tang) 《
长恨歌》:
上穷碧落下黄泉,
两处茫茫皆不见。(They searched the azure skies above and the Yellow Springs below — in both realms, vast and empty, she could not be found.) A Daoist priest is dispatched by Emperor Xuanzong to seek the soul of the dead Yang Guifei (
杨贵妃).
碧落 is the highest Daoist heaven;
黄泉 is the underworld.