http://www.youtube.com/watch?v=8LP7RjlrU6Y
yeah I misread that, 5 feet deep seems kinda overkill. Considering pros can get a monster plant out of a 200 gallon smart pot. I think they are only 18-24 inches deep.
Good luck on your grow. Post pics of that pit, if you go the mega hole method. I think that's what these guys did (link above).