Just a huge glass wall that acts solid and prevents mouse-over and movement through it, it’s invisible and cannot be seem, and you cannot move through or around it… but you can see it from outside the hold, flying above it and if you drop into it from above, it disconnects you because the game does not expect you to be in that space.
Blizzard what the heck did you do, is there a pencil tool in the dev kit to draw boundaries or something? How does something this gigantic happen by accident? D: