(CNN) — They stand watch over airfields around the planet, ensuring that tens of thousands of flights take off, land and taxi safely every day.
Yet airport control towers are often overlooked by air travelers -- or at least some would be if they weren't so weird looking.
Built by some of the world's most experimental architects, many belie their functional purpose by displaying an often stark beauty worthy of more than passing attention.
That attention has finally come, thanks to U.S. aviation photographer Carolyn Russo.
Russo, who works for the Smithsonian National Air and Space Museum, has recently completed a nine-year quest to capture 100 control towers in 23 countries, documenting their stark, sometimes surreal architecture in a series of powerful images.
She was inspired on her project by a close encounter in New York while heading home to visit her parents in Connecticut.
"I'm sitting on the plane looking at the towers and the tower at La Guardia came into full view.
"The texture of the tower was the creamy, vanilla-looking concrete with big circular windows. I just said, 'holy crap, it looks like Swiss cheese,' and at that moment ... it just hit me. I could bring these gigantic structures, these aviation artifacts, back to the museum."
East Fortune, Scotland: Now home to tractors.
Daunted by the scale of a project she thought was "a little nuts," Russo says she made limited progress until she found herself sitting next to a man wearing a U.S. Federal Aviation Authority badge at a Smithsonian dinner function.
"I asked him, 'Would you know someone I could talk to about getting permission?'" recalls Russo. "He just had this great big smirk on his face and said, 'That would be me.'
"I had no idea I was talking to the acting director of the FAA."
Russo began photographing towers across the States, combining her project with lecture talks and personal travel.
A trip to Finland then began opening doors to other airport towers across the world.
"Working internationally was fantastic," she says. "But it was a lot of work getting access.
"At London Heathrow I had to have an insurance certificate for $20 million just to be on the tarmac. LAX was also an expensive one."
Slowly, over the years, Russo's portfolio expanded as vacations and lecturing work took her to Geneva, Milan, Pearl Harbor, Australia and all points in between.
"I became obsessed with them," she says.
"From a personal point of view, I felt that they had a presence to them.
"When I went to photograph a tower I did a ton of research, but when you get there, it was not only trying to make a portrait of this tower it was like going out there and meeting an old friend."
"I viewed towers as symbolic, as cultural greeters," says Russo. "When you fly in, the first thing you see is the tower before you get to the city.
"I really did see them as guideposts, the choreographer of the airport. I don't like to say the word godlike, but I did see them as these intelligent structures keeping humans safe."
An exhibition of the images, ranging from gigantic concrete sentinels overlooking the deserts of the UAE to squat metal boxes in windswept Scottish fields, has recently wrapped up at the Smithsonian and is now going out on tour.
Russo says her photographs reflect how she anthropomorphized some of the towers.
"When I photographed Farnborough [west of London], to me it looks like a bird in flight, while Heathrow looks like a gentleman's top hat -- like a concierge at a hotel in London."
"For me personally I tried to read more into these towers, to elevate them beyond these architectural structures."
Her collection also includes decommissioned towers, with the hope that highlighting them will help conservation efforts.
"To me these are eyewitnesses to a bygone era of aviation, they're like these wise structures that have seen it all."
Among them is one of her favorites, a wartime naval air tower in East Fortune, Scotland.
"It's a low concrete building out in the middle of a wheat field and the farmer uses it to park his tractor.
"It's really beautiful and I found it by chance."