Ok, well I had a quick search around and couldn't find anything that was similar to this. I know that there is a bug to get under northrend using a Vehicle but all this requires is a flying mount! (and cold weather flying ofc)
Excuse the terrible editing skill with the video. My PC absolutely hates movie maker, which incidently was the only video editing software i had lying around at the time -.-
[YT][/YT]
Getting out is as easy as flying through the floor... upwards.
As for what can be done with this it is up to you, in icecrown there are plenty of places where you can land under the ground and attack mobs, although the mobs are able to come through the floor and come and attack you, PvP active players will not be able to. Find a decent spot where you can stand under the floor (a ledge or something) and greif to your hearts content.
Appologies if you believe that this should be in exploration, if you do believe that it should, feel free to move it. I put it here due to the fact im sure a hole in the world wasn't what blizzard had planned.