Tao Yuanming, a renowned writer of China's Eastern Jin Dynasty (317-420), authored the famous essay The Peach-Blossom Spring. It narrates the tale of a fisherman who chanced upon a hidden land. After squeezing through a narrow cave, he discovered a village inhabited by the descendants of refugees from the Qin Dynasty. This was a paradise isolated from the outside world, free from exploitation and oppression, where everyone lived and worked in peace and contentment. When the fisherman left and tried to return, he could never find the place again.
This idiom, derived from the story, refers to an isolated, utopian world.