The Grotto-Heavens and the Blissful Lands are worlds believed to exist hidden within famous mountains and beautiful places. They are earthly paradises that do not suffer from floods, wars, epidemics, illnesses, old age or death. Such imaginary places are usually known by the single compound, dongtian fudi. However, the two words originally referred to different things, fudi broadly meaning “paradise” and dongtian denoting an underground utopia.
One of the earliest descriptions of the Blissful Lands is found in the Baopu zi. The major mountains, says Ge Hong, “have gods of their own, and sometimes earthly transcendents (dixian) are to be found there too. Numinous mushrooms (zhi) and grasses grow there. There you can not only compound the medicines, but also escape war and catastrophe” (see Ware 1966, 94).
And, very interesting vis-a-vis Quatrian mythology:
Another feature of the grottoes is that, although each one is independent of the others, they are all linked in a network by underground passages called dimai (“earth channels”).