In the northern part of Darkshore there’s a massive whirlpool known as the Maw of the Void. Alliance players can jump down into the whirlpool and complete Ghosts in the Dark which allows them to discover the secret of the Maw. I’ve heard that you can also get the [Going Down?] achievement from this without taking fall damage. Try it during Brewfest to get completely smashed and get the [Drunken Stupor] achievement.
As a Horde player, I discovered this gem sometime last year. Jumping into it starts a short cutscene of your toon being sucked into the whirlpool. You end up in the Bashal’Aran Collapse where a lone Cenarian Circle druid sits by the waters edge.
Bielara is a Cenarion Circle druid who can teleport you back up to the surface
This is where you end up, in a cave above the Maw.
On a side note, I wondered if the huge whirlpool in Stranglethorn Vale did something similar… it doesn’t. All I got was a repair bill :P
You’ve been warned :)