Obdulia Delgado turned toward the on ramp of the Kennedy Expressway when she saw something in the middle of traffic that made her stop.
She saw the image of the Virgin Mary in a large yellow and white stain on the concrete wall at the Fullerton Avenue entrance last week.
"I was so stunned I couldn't move. People were honking," said Delgado, 31. "It was a dream. I don't even know how I got home."
By Monday morning, dozens had gathered to see what they believe is the image of the Blessed Mother on the wall of the underpass. Groups of people filtered past the site all day, some lighting candles and leaving flowers, others praying the rosary. Most snapped pictures with digital cameras and cell phones.
To some who saw it, the image appeared as a white outline of the Holy Mother's face wearing a shadowy cloak. To others, it looked like an ivory pawn from a game of chess.
As believers came to the spot throughout the morning, police put up temporary barricades to prevent people from driving and parking in the area on the north side of Fullerton Avenue.
Delgado said she had been praying to the Virgin Mary to help her pass a final in culinary school when she saw the image.
"There are many people here who believe in her. She's here for a reason," she said. "For me, it's not a watermark, it's the Virgin Mary."
It is not unusual for people to claim to see an apparition of the Virgin Mary or Jesus in unusual places.
In November 2004, a piece of popcorn shaped like the Virgin Mary was auctioned on eBay. A Canadian woman also said she saw the Blessed Mother and baby Jesus on a Lay's Smokey Bacon Chip. Thousands of Greek Orthodox flocked to Athens in 2001 to see a "bleeding" Virgin Mary statue.
For now in Chicago, the image will be allowed to stay on the wall, surrounded by less identifiable water stains and paint marks.
"We're treating this just like we treat any type of roadside memorial," Illinois Department of Transportation spokesman Mike Claffey said. "We have no plans to clean this site."
Apparitions of Mary hold different meanings for believers, but people may draw connections to current events, like the death of Pope John Paul II, said Cristina Traina, an associate professor of religion at Northwestern University.
"Most often, the people who see the image interpret it as a sign of affirmation of an event or behavior or a condemnation of an event or behavior," she said. "What is miraculous is that a natural event like a stain from leaking water and a supernatural event like seeing Mary converge."
Michael Grzesik, who leads people on religious pilgrimages, said that when he first looked at it, he saw nothing unusual.
"I was looking at it and thought it might be an oil spill. But as I got closer it resembles Our Lady," he said. "It really struck me . . . I think Our Lady is always with us, and this is another sign she is with us."
Grzesik compared the image under the expressway to an apparition of the Virgin Mary that appeared more than a century ago in a grotto in Lourdes, France.
"This is like Our Lady appearing in Chicago in a grotto under the Kennedy," he said.
But he was also light-hearted about the image: "There's a little graffiti around that says `Go Cubs,' so it looks like Our Lady is rooting for the Cubs."
The Archdiocese of Chicago has not received any requests to authenticate the image, spokesman Jim Dwyer said.
"These things don't happen every day," Dwyer said. "Sometimes people ask us to look into it. Most of the time they don't. [The meaning] depends on the individual who sees it. To them, it's real. To them, it reaffirms their faith."
Victor Robles, 36, who got a close look, remained skeptical.
"I see just a concrete wall and an image that could happen anywhere," he said. "It makes me feel good that there are people with faith . . . If that image helps more people feel closer to God than maybe that is a good sign."
Irene Munoz, 30, walked past the crowd before deciding to see what everyone was looking at.
"It's very emotional," she said. "It's very real. I never believed anyone who saw these things. But I believe now."
As word of the image spread, a teacher from Holy Trinity High School sent students to look.
"If you look, you see her face popping out and the veil and her hands," said 17-year-old Luis Flores. "That's the image that's portrayed in the Bible. Many miracles have happened, but this is one that just appeared."
Some of those who gathered felt the appearance of the image had special significance as the papal conclave meets in Rome.
"It's amazing it's the same day they're picking a pope," said Juan Soria, who rushed to the site with his family. He saw the image as "a message from above. It's a cry for peace and hope to get rid of tyranny in the world."