Riverworld

Riverworld is a fictional planet and the setting for a series of science fiction books written by Philip José Farmer (1918-2009). Riverworld is an artificial environment where all humans (and pre-humans) are reconstructed. The books explore interactions of individuals from many different cultures and time periods. Its underlying theme is quasi-religious: who did this? Why? How? The motivations of alien intelligences operating under ultra-ethical motives are explored.

This concept is similar to the one described in the short story of Robert F. Young called "On the River" (1965).