Mammatus clouds. The shot is taken at Chalandri, Athens after an unusually intense thunderstorm.
Canon EOS DIGITAL REBEL XTi, 4/7/2009 4:29, Shutter Speed 1/500, Aperture Value 11.0, ISO 400, Lens EF-S17-55mm f/2.8 IS USM, Focal Length 24.0 mm