The slab found off the rocky islet was originally a step in the large, central staircase of Tiras Maegthil; it is currendy sealing the entrance to the tower's basement. Moving the slab releases a bubbling froth of foul air as the chambers beneath begin to fill with water. How fast the ruins fill depends upon how much the stone has been shifted. The air inside is completely unbreathable, but if a dam can be built around the slab, the basement area just below will be passable in less than an hour.