Abstract -- An unusual image was photographically recorded by an official mapping aircraft of the Costa Rican government at 08:25 am (EDT) on September 4, 1971 while flying at 10,000 feet altitude over a body of water known as Lago de Cote. None of the flight crew or photographers saw the object. Second generation negative and positive black and white transparencies were obtained and analyzed by the authors. Both transparencies were photographically enlarged and printed on various contrast papers for purposes of making visual inspections and linear measurements. Computer enhancement showed variations in surface brightness. The preceding frame, taken 20 seconds earlier of the same ground region, did not show the disc. The angular position of the sun was determined for the date, time and location of the event and was found to be consistent with cloud shadow positions but not with the dark regions on the disc. A shadow of the disc could not be found. The oval image measured 4.2 mm on the negative and was enlarged to 41 mm (9.76 X magnification). If the disc was located 10,000 feet away from the camera, its maximum dimension would be 210 meters (683 feet). The various analyses failed to identify the image. The same body of water was the site of a visual observation of a partially submerged object on October 25, 1986.

On September 4, 1971 a mapping aircraft of the government of Costa Rica with a crew of four recorded an unusual disc-shaped image as it was flying over the region of Arenal. It took several years for this photograph to find its way into the hands of a Costa Rican investigator, Mr. Ricardo Vilchez who