A lake with a house on it.
I wish I could remember the whole dream now, as there was so much of it, but all I can recall is the house hovering just over the surface of the lake. We swam in the lake and pulled ourselves up into the house halfway through the dream. And of the inside of the house, I can only remember one thing-- that it was haunted, no, possessed by a sad and hateful spirit. At one point, I heard knocking from across the house. I strode over to the large double doors into the dining room and threw them open, ignoring whoever it was that was with me shouting that I shouldn't do it, and it was there-- invisible and angry. It tore through me as it careened through the house, and it hit like a baseball bat in the stomach, all light and sound and visceral impact. I would have thought that would have woken me up, but it did not-- the dream continued, though I cannot remember what happened next.