Not sure how old this is, but it's new to me. If you fall into the swirling vortex in Darkshore it will port you to a small cavern named the "Bashal'Aran Collapse." There is a demon hunter inside, as well as an NPC to port you back out:
I checked the vortex in Stranglethorn, to see if there was a similar cavern. Alas, it's just normal water at the bottom:
They may add something there in the future, though, since it seems pretty silly not to. With the vortex in Vashj'ir leading to the Abyssal Maw, and now the one in Darkshore leading to a cave, it seems only reasonable that the STV vortex will lead somewhere as well.