Which of these was the principal river of the underworld in Greek mythology?

When you cross the river styx, crossing from the world to the underworld, be sure to have payment for the ferryman Charon. This is where the practice of leaving coins on the eyes or in the hands of the deceased began.

I have swum in the River Acheron, mainland Greece, which is, supposably, one of the rivers of the underworld. It is fed from the mountains and is very cold, even in high summer. Very invigorating.

Sassy, I saw Styx in concert twice back in the 70's/80's.....awesome!!

Just like the rock band styx