...and appropriate for this holiday (Oct 31st) season:

I work part time in a gym that has a gorgeous Olympic-sized HEATED swimming pool. Every once in a while, the heat will go out in the room housing the pool (Yes, I know, "natatorium," but some of the muscleheads I deal with barely understand the concept of "swimming pool") but the water heater will continue to function. If this occurs on a cold winter's day, a very thick and spooky fog will form over the water and surrounding deck area to a depth of up to two feet. When this happens, we rename the pool "Lake Transylvania" and the skinny-dippers have a field day!