Are we alone? Not according to those who celebrate World UFO Day.

Each year on July 2 extraterrestrial enthusiasts around the globe celebrate unidentified flying objects and the possibility of life outside planet Earth.

USA TODAY Network looks back at some of the most buzzed-about sightings of unidentified flying objects.

1. Nuremberg, Germany, in April 1561

Witnesses described seeing "globes," "crosses" and "plates" battling it out in the sky. Drawings and woodcuttings from the time depict the unusual sighting.

2. Aurora, Texas, in April 1897

Local lore has it that a cigar-shaped object allegedly hit a windmill, causing it to crash. An alien body was discovered in the wreckage and buried in an unmarked grave, according to legend. The Texas Historical Commission erected a sign that mentions the "legend that a spaceship crashed nearby in 1897 and the pilot, killed in the crash, was buried here."

3. Roswell, N.M., in July 1947

Roswell has become synonymous with UFOs. A rancher discovered debris from a crashed airborne object. Some people said they thought the debris came from a spacecraft and even saw alien bodies.

The Air Force said in a 1994 report that the material came from experimental surveillance balloons and any bodies were dummies in the balloons. But many remain convinced the government covered up an extraterrestrial visit.

4. Washington, D.C., in July 1952

A series of sightings were reported over the nation's capital. Washington National Airport and Andrew Air Force Base detected the objects on their radar, according to The Washington Post.

In 1952, the Air Force launched Project Blue Book to collect and analyze potential threats in the air. Air Force Maj. Gen. John Samford held a press conference at the end of July 1952 to address unidentified sightings, opening with, "I am here to discuss the so-called flying saucers."

The incident has since been attributed to temperature inversion.

"The warm air can create a ceiling that causes radar beams to bounce down. Objects on the ground — moving cars, a row of telephone poles — can appear to be thousands of feet in the air," the Post reported in 2012.

5. Chicago, Ill., in November 2006

An object was spotted hovering over Gate C-17 at O'Hare International Airport. Some witnesses told the Chicago Tribune they were upset that neither the government nor United Airlines, which operates the C terminal, was going to investigate the sighting.

6. Stephenville, Texas, in January 2008

Dozens of people said they saw a large, low-flying object with bright lights. Two nearby Air Force bases said that none of their aircraft were in the area.

7. Jerusalem in January 2011

Witnesses and at least three cameras reportedly spotted a bright light hover above the Dome of the Rock in Jerusalem before suddenly shooting upward. The sighting was likely a hoax, said an Israeli TV station.