Not sure if this is a bug or glitch or just a clipping problem, but I fell under the world while battling the spawn in the cave near the Meremoot skill point in Kessex Hills. In combat as usual, inside a cave mind you with no water anyway, then suddenly I’m in the water! WTK? Took me a moment to realize what happened.
It would appear that the GW2 “under world” is an infinitely deep ocean that one cannot escape, down, up, sideways, anything. I did find, however, that you can revive players and NPCs above you, lol, and that you can even target and attack mobs. I was not exploiting the issue, just poking around a bit to find out what was possible. One guy that I attacked actually surprised me and managed to get down under the world with me!
ArenaNet you should put some truly scary monsters down there just to make it interesting whenever this happens.
Below is a short video I shot while swimming around under the world for anyone interested.