21C MYSTERIOUS ISLAND EXIT. Inquisitive fish will peek at adventurers through a thick membrane which separates this passage from the lake water outside. It can be easily penetrated by any person wearing a bubble, but will impede the progress of all others. It can be destroyed by doing a combined 50 points of damage in a single combat round, but that will allow the lake water to flood the complex. The bottom of the floor is about ten feet below the lake surface, and the passage to the water is about six feet high, so there is another four feet of lake between the top of the membrane and the surface.