Evidence of a mysterious craft hovering over a lunar horizon was recently discovered by a UFO researcher in an archival photo from the 1971 NASA moon mission Apollo 15, Inquisitr reported.

Although UFO enthusiasts were warned that the sighting did not mean it was definitely a UFO, the researcher hinted that it could possibly prove that an extraterrestrial observer was monitoring the world's fourth trip to the moon.

"Dudes, I just upload what I find, I believe it's put there to be found," said the researcher who goes by the YouTube screen name Streetcap1. "Remember a UFO is an unidentified flying object, not an alien craft."

The photo, taken on the surface of the moon between July 30 and August 2, 1971, could have been clicked during the 67-hour time period that astronauts David Scott and James Irwin were on the lunar surface.

However, some critics remained skeptical about the origins of the mysterious craft.

"The object... does appear to be some kind of craft, but with a shape unlike anything I have seen before," Scott Waring, whose blog UFO Sightings Daily catalogs a voluminous number of UFO sightings worldwide, wrote last week.

"Sure it looks like a triangle, but actually it seems more like three large tubular sections that become one. This is probably a drone keeping an eye on the Apollo 15 mission, perhaps out of curiosity, or perhaps out of fear," the fellow UFO researcher added.

"It's certainly one of the more unusual UFOs we've seen," wrote Thomas Tamblyn in the U.K. edition of the Huffington Post. "Of course the real likelihood is that this is just a fragment of dust on the camera lens or a developing error."

But five days after Streecap1 posted the photo, Waring also pored through a stack of NASA Apollo photos and found what he claims is evidence of a UFO monitoring a human space flight from the 1969 Apollo 12.

The UFO "was probably circling the area to observe the mission," Waring said.

Meanwhile, detailed enhancements of the 44-year old alleged UFO NASA photo can be seen in Streetcap1's video here: