I have finished Earthen Peak (and drained the poison pool, of course). Now I was returning there on a whim and I noticed there is a pool of poison with loot and player messages. It's the pool where you can run just to the right, get ambushed by a Manikin and open the first Pharros lockstone of Earthen Peak.
How is it possible to get there? I can see player "shadows" running around in the pool so clearly it must be able to get there.