The underground river is located within Underdark and is found at the bottom of a waterfall in the Abandoned Mine.
| Spoiler: This is the lead in to Underdark after the Princess and her party jump over the waterfall. The rough landing may leave some party members dead and others will go missing. If Gemli is in the Princess' party, his remains can be found on the other side of this area, which is not accessible from the starting position. |