rectangular shaped object over Oakland, CAI was walking on my boat dock in Alameda, CA when I saw a rectangular shaped object traveling over the City of Oakland, CA. The object was translucent with a haze around the edges. At first I thought it was a large piece of plastic floating in the air, but it maintained its shape and course. The object floated up and down changing altitude about 100-200 feet each way. The object started in the area of the Mormon Temple and disappeared for several seconds in low rain cloud before emerging and continuing west. I lost sight of the object around the vicinity of the Bay Bridge. Approximate altitude was 1000 feet based on the height of the East Bay Hill in the background. It appeared to maintain its heading throughout the sighting. No lights or other reflections were observed. This area is in the vicinity of the Oakland International Airport, but I did not see any other aircraft in the area.