The Abyss is one of our two Gateways. It's counterpart is The Hole in the Sky. Unlike the Hole in the Sky, the Abyss does not have a presence in The Mythos of the Ktletaccete. It does, however, correspond with the Engine Room as part of our inworld construct. In our novels, this would be the sun intake of the aft endcap of the Sunspot's habitat cylinder.
In our actual inworld, it is a gigantic, inky black pit at the very bottom of our psyche. Jenifer and Eh perceive themselves to have come from it, or to even be extending up and out of it still connected to it somehow. And Phage treats it as its headspace, its home in our inworld.
We have tried exploring it repeatedly, but cannot seem to go deeper than being surrounded by complete blackness. The very moment one of us who is visiting it turns upward, we leave it.
One of our theories is that it is not a true gateway, but a psychic construct that represents time before our existence. Another theory is that we cannot go back through it because that would involve going backward in time.
With Project Open Door, we are exploring the Hole in the Sky and the space beyond it. And we hope that perhaps what we learn from doing that will teach us more about the Abyss and what we might be able to do with it. We do not expect useful results, though.