I'm pretty sure I'm the first one to discover this so here goes nothing.
In-between the bottom of the ocean, the end of the world killplane and the ocean there is an airpocket that you can stand in. This airpocket is around 30meters long and 10 meters high and stretches along the entire rim of the world.
In said airpocket you are able to walk around, build, dig, etc. Nothing can spawn down here, campfires ( and their variants ) torches ( and their variants ) do not function, Nothing grows down here (the game thinks its an ocean biome), but aside from that its a regular location. I accidently discovered this location while using debug mode to fly around and check out the out-off-bounds biome, and accidently discover this while flying around in the ocean.
So is it possible to get down here? The answer is yes. Your going to need a friend an elevated slingshot that's above y level 110 (to get the necessary downwards velocity to get through the ocean )
After you have all this use said slingshot to launch your friend off and hopefully he lands inside the airpocket ( this is gonna take a few attempts ) once their in have them set up a portal and your good to go.
While in the airpocket make sure to not go to high, the ocean will suck you up and you will most likely drown. avoid the killplane ( it kills you ).