In the early, blue-sky area of the game, there's a room with a stone arch and two treasure chests. One of the chests is on top of the arch, but the other is on a floating ledge far to the side. This is the room Gomez enters at 3:35 in Polytron's trial gameplay video, though the ledge doesn't appear in the video (I guess it's only in the full game?).
Once you get over to the floating ledge, how do you get back down? It looks like all the ledges you can jump to are too far down to survive, and this room doesn't have water.