That's a puzzle to solve. The topic here doesn't include a spoiler warning, so that's all I'll post visibly.

[spoiler]
The cave is uncovered by moving the rubble to the right of the bath. Inside, part of the image shown will be of a woman holding a torch to illuminate a wall, which will have hieroglyphic-like characters on it; they're potion recipes. One for magic, one for nature. If all you want is the potions, then go to the guides forum, and check out the potion list.
[/spoiler]