After completing the quest in Briarthorn Den, I exited the Den from the right exit. I was then spawned under the world. I was swimming around in water but could not get back through the ground until i warped to a waypoint. I was able to attack enemies through the ground. If you need any further information please let me know.
I was on my lvl 13 Sylvari Hunter. I have a short video if you need it please let me know and i will send it.