No. It's not a UFO! The mysterious object at the center of the photo is just an overexposed ship, traveling across the sea at Sounio, Greece. The faint glow to the left at the edge of the hill comes from the temple of Poseidon. At the same side, a distant thunderstorm is visible. Note also that the man on the picture casts two shadows. One from a dim red light behind the camera and the other from the rising Moon.
Canon EOS DIGITAL REBEL XT, 22/11/2008 05:32 UT+2, Shutter Speed 30 sec, Aperture Value 2.8, ISO 800, Lens EF-S17-55mm f/2.8 IS USM, Focal Length 17.0 mm, 1 light, 5 darks, 5 flats, Processed at DeepSkyStacker, Photoshop CS2