It’s a bird, it’s a plane — it’s a … UFO? Probably not. But there are thousands of recorded alien-like sightings a year in the U.S. And there tend to be commonalities.

They occur around government laboratories or military sites

Thousands of UFO sightings have been reported around Los Alamos National Laboratory in New Mexico, for example. The most notable of these sightings are the so-called “green fireballs,” which have been controversial since their earliest sighting in the 1940s. Astronomer Lincoln LaPaz was assigned by the U.S. government to investigate the occurrence in 1948. Upon seeing them for himself and gathering observations from locals, he concluded that there were too many anomalous factors to consider them meteors or aliens. To this day, there is no known explanation.

In 1965, at the Edwards Air Force Base in California, another strange incident occurred. A tower operator at the base spotted about a dozen flashing red and green lights zipping through the sky. The operator contacted officials at the lab and documented details of the incident, including a 40-minute recording of the exchange between the operator and various officials. The lights persisted in for six hours and eventually ascended into the atmosphere, blending in with the starry night sky. Dozens of local reports cropped up describing eyewitness encounters with the flashes.

The incident was once again, unexplainable. But, it was filed in Project Blue Book, an Air Force investigation that examined two decades of UFO sightings. The project was shut down after four years on the basis that the so-called sightings were not a threat to national security.

They defy the laws of physics

Sir Isaac Newton’s first law of motion: An object at rest stays at rest, an object in motion stays in motion, unless acted upon by an external force. Gravity simply does not allow hovering, bouncing, or zipping of anything in the sky. Yet, most UFO sightings involve some kind of physics-defying property.

The National UFO Reporting Center has a record of nearly all alien sightings in the U.S., though they do not make any claim to the validity of aliens on Earth. On the NUFORC database there are accounts from people in every state who have apparently witnessed an unidentifiable object.

In 2002, a police officer in Montana caught a glimpse of a “floating” object over the highway. It made no noise and had no lights of its own. In 1995, an Alaskan woman driving to work was suddenly engulfed in fog and saw a ship “hovering” over her car. In 2013, in New Hampshire, a family drives on the highway at night and spots orange balls of light just before they “vanish.”

There’s a problem with the footage

In an age of Instagram and smartphone cameras, taking photos is practically second nature. Yet, when it comes to taking a simple photograph of a UFO, technology almost always fails us. The objects are too far away, moving too fast, or cannot be detected by cameras. We very rarely see an image that can definitively prove that aliens exist. If a photo is captured, the image often turns out to be a weather balloon or a satellite. Sometimes, it’s nothing more than a funny glare or reflection. Other times it’s a weather phenomenon, like these lenticular clouds in South Africa that freaked everyone out.

There are still some major thresholds to cross before humans will ever believe they aren’t the only intelligent lifeforms in the universe. For now though, it’s safe to assume we are, in fact, alone.

Photos via Flickr / Hood Ornaments