What if spiders could fly? In Chicago, perhaps they do.

Every year, Chicago is inundated with "flying" spiders, and they're not the only city. How do spiders do it?

It’s a bird! It’s a plane… It’s spiders?

In Chicago, spring brings warmth, budding plants, and an invitation to enjoy the outdoors. However, it also brings with it what some may consider their worst nightmare: “flying” spiders.

While it may seem impossible, every spring arachnids take to the skies, seemingly defying their biology. It happens in Chicago and it happens worldwide, becoming one of the most interesting phenomena surrounding the eight-legged creatures.

Every year, Chicagoans face the spring with windows closed and eyes to the sky. Maggie Gellens, a Chicago resident, says it is an annual event.

“Every spring we start to see a couple of spiders on the outside of our windows, sometimes it can be a lot. I don’t know how they get up there because we live on the 81st floor,” Ms. Gellens told FOX 32 News in Chicago.

Hotels have also taken to notifying guests that they should keep their windows closed, unless they want an unsolicited visit from an eight-legged guest. So where are these flying spiders coming from, and how do they do it?

It has less to do with supernatural ability and more to do with science. Steven Sullivan, the senior curator of urban ecology at The Peggy Notebaert Nature Museum, said spiders travel through the air with a method called “ballooning.”

“Ballooning is when they simply let a little strand out of their abdomen, [it] gets caught by the wind and they blow along until they happen to hit a place like this, where they can build a web,” Mr. Sullivan told FOX 32 News.

Ballooning spiders are not only a Chicago phenomenon. In 2013, northern Texas experienced a similar influx of ballooning spiders. Patrick Dickinson, a horticulturist at Texas A&M University's AgriLife research center, explains that it happens worldwide.

“In Brazil they do it this time of year when they look for the flight of the spiders, and the skies will literally be clouded by the shrouds of these strings from spiders,” Mr. Dickinson told CBS DFW.

The phenomenon occurs in spiders around the world, often when they are babies. After hundreds hatch from their eggs, they need to disperse and find a habitat to survive. Ballooning allows them to get quickly to a variety of locations. It happens in Chicago largely due to the insect breeding that occurs around Lake Michigan, reported FOX 32 News.

Jim Louderman, a collections assistant at the Gantz Family Collection Center who specializes in spiders, told the Chicagoist that the spiders found ballooning in Chicago are harmless. They are less than 3 millimeters long, and as babies do not have enough venom to cause any damage. About 50 species of spiders can be found in Chicago, with the majority being Larinioides sclopetarius, nicknamed “High Rise Spiders.”

LiveScience reported that ballooning enables spiders to travel hundreds of miles. Some ballooning spiders have even been found on islands located in the middle of the ocean. They are left to the whims of the wind, with little control over where they travel. Since many species are attracted to hard, rocky surfaces, skyscrapers make a perfect home for urban dwelling spiders. Mr. Louderman said that in collecting spider samples in Chicago, they have found the arachnids on the top of the Willis (Sears) Tower, 108 stories high.

Dickinson said it is an annual occurrence, and that it is actually beneficial for everyone in the area, humans and spiders alike.

“They’re going to be feeding on the bad insects that we don’t want. These are the good guys,” Dickinson said.

 [Editor's note: The original story misstated Steven Sullivan's title. Mr. Sullivan is the senior curator of urban ecology at The Peggy Notebaert Nature Museum.]