The Plug

The Plug is a key location in the quest to either A) remove Corruption from The Waters/ The Stem and save the world from rotting and withering away, or B) to somehow make the world worse/ give Aboleth/Father a bunch of food. Either way, Sigrid Arnulf made a deal with Father to unplug the waters in return for him helping her protect life. It's Probably Fine. Father's previous conduit seemed to think unplugging the waters was a good idea. Unlike The Sphincter between Plunge Pool Cavern and The Waters/ The Stem, this plug appears to be in The Waking and affects passage from The Waters to The Tree

"But the waters could be unplugged, yes. The filth could be removed. The plug. The filth. The excrement. It is on your side of the leaf, Daughter. Dive in. Take a look." - Aboleth/Father to Sigrid

When Sigrid accepted the vision by submerging into a river, she saw Land mammal skeletons covered in black ichor, seemingly at the bottom of the lake. The ichor appeared to be the cause of Corruption's spread, as water within the lake feeds into the land and rain and travels over the world.
It is unclear whether the plug truly is at the bottom of Ael Nurnan lake, or how the giant bones (or the ichor on them) can be removed.