Underworld River

Explanation: Styx is a small natural satellite of Pluto that was discovered by a team led by astronomer Mark R. Showalter, using fourteen sets of images taken between 26 June and 9 July 2012 by the Wide Field Camera 3 fitted to the Hubble Space Telescope. The convention for naming Plutonian moons is to use names associated with the god Pluto in classical mythology. The Styx is a river in Greek mythology that formed the boundary between Earth and the Underworld.

Pluto

Image credit: Wikipedia.