BOSTON -- In a canyon walled by drab grey and brown high-rises, a ragged line of men, women and children hold red helium balloons, ascending a staircase that ends in a spiral at an opening in the roof, where a single giant balloon slips into the blue sky. Two people at the top lift their heads in wonder.

Seen by thousands of eyes a day at one of Boston's busiest intersections, the giant outdoor mural by Iranian artist Mehdi Ghadyanloo is a stunning expression of optimism he hopes can lead to better understanding between his homeland and the United States.

The 76-by-70-foot mural, painted on the side of a building across the street from South Station, appears almost three-dimensional, as if the admirer can join the line and find out what great secret awaits above.

"I wanted to create something hopeful, especially now you always you see so much bad news from all around the world," Ghadyanloo said. "I think what we need as humans is hope to keep going and enjoy our lives."

It's the fifth mural at the site commissioned by the Rose Fitzgerald Kennedy Greenway Conservancy , which oversees a narrow, 1.5-mile-long strip of land created when the elevated highway that used to run through the city was torn down.

Iranians and Americans don't really understand each other, Ghadyanloo said.

The U.S. has not had diplomatic relations with Iran since 1980, the year after 52 American diplomats were taken hostage at the U.S. Embassy in Tehran. Ghadyanloo, 35, had not even been born yet.

But he wants Americans to know that despite political tensions, Iranians and Americans are not that different. They have dreams, they yearn and they love. His art, he hopes, can explain that.

"I think art is the international language," he said. "It's like a bridge between all cultures."

Ghadyanloo studied hospital administration at the University of Tehran but found it boring. He soon discovered art was where his passion and talent lay.

After graduation in 2004, he answered an open call from the Iranian capital's beautification bureau to submit proposals for murals to brighten the city. He ultimately painted more than 100 between 2005 and 2012.

But he became weary of painting murals, until he was online and saw a previous mural on the same Boston building -- which is actually an air intake for a subterranean highway -- by the Brazilian twins known as Os Gemeos.

"I said, 'This is the wall that can be next for me,"' he said.

Across the ocean, Lucas Cowan, the Greenway's public art curator, was already an admirer of Ghadyanloo's work.

"I'd been following him for about two years and when I contacted him through Facebook to see if he'd be interested in doing something in Boston I was thrilled he already knew the space," he said.

"Spaces of Hope" will stay up for a year before being covered up by another mural, and Ghadyanloo accepts his work's fate.

"At least I can have a little bit of a positive effect on everyone's life," he said.