The subject of myth and legend since time immemorial, its existence was confirmed by the Moderator Glen in November 2012, shortly before the scheduled end of the world. It is the dark howling abyss that is the abscess in the soul of AH.com. It is a place of horrors and madness. It is not for mortal eyes. Only the moderators and IAN may look upon it and hope to stay sane.
Pray you are never called upon to stand between the board and the sucking sibillant terror of the deleted!
In the religion of the Vexilologist Guild from AH.com Enterprise, the Underboard is the equivalent of Hell or the Underworld. According to teachings of The Guild, The Underboard is populated by the souls of n00bs who couldn't be bothered to save their maps in .png instead of .jpg format.