Police have patrolled the emergency turnoff area under the Kennedy Expressway since Monday as hundreds of people have walked down to see the image and the growing memorial of flowers and candles that surround it.
Beside the image is an artist's rendering of the Virgin Mary embracing Pope John Paul II in a pose some see echoed in the stain.
"We believe it's a miracle," said Elbia Tello, 42. "We have faith, and we can see her face."
Tuesday morning, women knelt with rosary beads behind a police barricade while men in work shirts stood solemnly before the image, praying. A police officer kept the crowd of about three dozen from getting too close to the traffic but didn't stop them gathering around the stain.
The stain is likely the result of salt run-off, according to the Illinois Department of Transportation. The agency does not plan to scrub it off the wall.
"We're treating this just like we treat any type of roadside memorial," said IDOT spokesman Mike Claffey. "We have no plans to clean this site."
The Roman Catholic Archdiocese of Chicago had not received any requests to authenticate the image as of Monday, 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."
But onlooker Victor Robles, 36, said he was skeptical about the stain's Virgin Mary resemblance.
"I see just a concrete wall and an image that could happen anywhere," Robles said. "If that image helps more people feel closer to God than maybe that is a good sign."
Worldwide, people have been drawn to images believed to resemble the Virgin Mary seen on windows, fence posts and walls.
Among the best-known in the United States was an image seen in office windows in Clearwater, Fla. Within weeks, a half million people had been to the site.
Glass experts believe the image was created by a chemical reaction and corrosion of the metallic elements in the glass coating, but they could not explain why it took the shape it did. The windows were broken last year.