Possible Clues:

• 'The Divine Comedy' river
• The French article produces Greek final solution to forgetting reality
• Hades river of forgetfulness
• River of myth where one drinks to forget
• River of the underworld in Greek mythology
• Water underground comes from toilet here
• In Greek mythology, a river in Hades
• Discovered in bundle theory - the flowery underground segment that helps you forget your life
• Underworld river causing forgetfulness
• One of the five rivers of 56-Across