Papyrus Riddle: Underground Currents
Find this riddle in the Tomb of Battos, in Cyrene, Kyrenaika.
"South of Cyrene, in the Green Mountains, there is a lake whose water comes from a magical stream. I'll be at the source of that magic, where water flows from nothing."
The lake spoken of is the lake of tears you sought during the Halo of the Huntress side quest.
Follow the waterfalls west.
At the very top you'll find the solution.