Earlier today i tried to teleport to someone who was trying to get a engineer's toolbox socket working in the deep dark. I ended up in a pool of lava for my trouble. Trying to tpa from over-world to someone in the deep dark results in you going to the overworld at aprox the same coordinates as the person deep dark. (Y values can differ i am guessing closest air pocket at least it is not putting you in the terrain) Tested that if you are both in the deep dark you can teleport to each other just fine. Same if someone in the deep dark teleports to someone in the overworld. Tested this with /tpa and /tp