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.
To empty the poison pool, you need to burn the windmill near the second last bonfire. You can do this by using a flame butterfly, or the torch (you can light it from the bonfire).
Once you do so, any pools of poison will be drained, including the boss room. There are several corpses that can be found in the pool, and the boss will lose her ability to regenerate. (There is still a ring of poisonous liquid around the edge of the boss room however).
As per your edit, that particular "pool" is inaccessible - basically; it's a killzone. If you can see player "Shadows" jumping in there, they are leaping to their deaths.
There is a small ledge off to the side, that leads to a small area full of Manikins that you can jump to, with a very well-aimed jump, but that pool of poison is not meant to be entered.
Did you burn the windmill? According to this video, you need to burn the windmill towards the beginning of the level to clear the poison.
You can see where to burn the windmill here. Looks like you'll need a fire butterfly.
