Rooted in Li Bai's Tang-dynasty poem 《
山中问答》: '
桃花流水窅然去,
别有天地非人间' — 'peach blossoms drift off with the stream; there's a world apart, not of the mortal realm.'
洞天 is also a Daoist term for grotto-paradises of the immortals, which adds the 'cave-heaven' color.