世外桃源 (shì wài táo yuán)
Tao Yuanming (táo yuān míng 陶渊明), a famous writer of the Eastern Jin Dynasty (dōng jìn 东晋,317-420AD), wrote the well-known essay Peach-Blossom Spring (táo huā yuán jì 桃花源记). In it he tells a story which goes like this: A fisherman happened to come upon a place called Peach-Blossom Spring. Squeezing through a cave, he found a village, the residents of which were descendants of refugees from the Qin Dynasty (qín cháo 秦朝). It was a paradise isolated from the outside world, without exploitation or oppression, and everybody living and working in peace and contentment. The fisherman left the villagers and went home. But he could never find the place again. This idiom is derived from the above story, and is used to mean an isolated, ideal world.