The photo, which has a July 5 time stamp, appeared on the Mutual UFO (Unidentified Flying Object) Network website last week and was allegedly taken by a security camera at an oil drilling site in Eagle Ford, the San Antonio (Texas) Express-News reported.
Charles Stansburge, a veteran MUFON investigator, said the photo has passed two authenticity tests.
"If it's a prank," Stansburge said, "someone spent a lot of money to stage it. It's not a doctored photo. It's a 60-foot-diameter saucer that's hovering."
Other unexplainable sights have been seen in the Eagle Ford-area, including a number of oil workers and nearby residents seeing unidentified lights in the night sky on consecutive days in October.
Stansburge said he is sure what his investigation into the photograph and other sightings in Eagle Ford will uncover.
"All we can do," he said, "is document what we find."