Bsftcs has pointed you in the right spot. Go north from Ishad N'ha, past the burnt-out house to the lake. Swim north through the lake to a large clearing on the other side. Go further north until you're almost off the map, and you'll find a hill. The entrance is only visible from the north, so you'll have to go beyond it and come back.
And after you flip, be ready to jump...
