UFO skeptics say they can't be alien ships because the vast distances of space are just too much.
But a university professor in the US thinks he's solved that problem - they're not from other planets at all. They're piloted by humans, but from the future.
"We know we're here. We know humans exist," professor of biological anthropology at Montana Technological University told space.com last week.
"We know that we've had a long evolutionary history on this planet. And we know our technology is going to be more advanced in the future.
"I think the simplest explanation, innately, is that it is us. I'm just trying to offer what is likely the most parsimonious explanation."
Rather than being 'extraterrestrials', Dr Masters suggests we're actually witnessing 'extratempestrials' - humans from the future who've figured out time travel and have ships capable of pulling off moves that ours can't - such as those seen in the infamous 2004 US Navy video.
Some come back to do experiments and learn more about their distant ancestors, Dr Masters proposes.
"The alleged abduction accounts are mostly scientific in nature. It's probably future anthropologists, historians, linguists that are coming back to get information in a way that we currently can't without access to that technology," he explained.
Others might just be tourists.
"Undoubtedly in the future, there are those that will pay a lot of money to have the opportunity to go back and observe their favorite period in history. Some of the most popular tourist sites are the pyramids of Giza and Machu Picchu in Peru - old and prehistoric sites."
Dr Masters outlined his theories in a book last year, Identified Flying Objects: A Multidisciplinary Scientific Approach to the UFO Phenomenon, which is rated four-and-a-half stars on Amazon.
His research has predominantly been on how humans' heads have changed shape over time. Aliens are commonly depicted having much bigger heads than us, presumably to hold bigger brains.
Dr Masters suggests the trajectory of human evolution means they could feasibly be us, just many years from now.
"The book ties together those known aspects of our evolutionary history with what is still an unproven, unverified aspect of UFOs and aliens," he told space.com.
