The Shimmering Portals

From the introduction: The player characters enter from the trap door above the center of the room. If they fall, they take damage from the 10’ drop. Otherwise, they can climb down normally. It is a plain stone room, 15’x15’, with a stone floor and a stone ceiling. There is a doorway in the center of each of the four walls, with no door but a bluish, shimmering light like reflections on water filling the space.