The human-sized yellow and white image appears to have formed from road salt and rainwater that over time has spilt from the Kennedy Expressway overhead. Nonetheless, believers insist that the stain is a miracle.

The underpass has been transformed into a shrine, bearing flowers, candles and an artist’s impression of the deceased Pope embracing Our Lady — an image that some see in the stain.

As the faithful knelt before the wall yesterday, praying and clutching rosary beads, authorities promised to protect the concrete wall as long as road safety was not compromised. Police have sent out regular patrols all week to monitor the crowds, which swelled into the thousands late on Tuesday.

Patrol cars are more used to using the location as an “accident investigation site”, where they question speeding drivers.

Matt Vanover, spokesman for Illinois Department of Transportation, would not be drawn into a theological discussion of the image. He did note, however, that his department had had to close part of a nearby highway exit.

“I’ve only seen photographs,” Mr Vanover said, “but it’s my understanding that it’s more visible in low light.”

The pilgrims were in no doubt, however. Some used the camera function of their mobile telephones to capture the image more clearly.

“We have faith, and we can see her face,” Elbia Tello, 42, said of the image, which a Chicago Tribune columnist has named “Our Lady of the Underpass”.

One visitor to the underpass, Frank DePaul, said: “I am drawn to the crucifix which is hanging from her hands and it's coming down the middle there. And I’m seeing a rosary which is a powerful prayer in our Church.”

The Roman Catholic Archdiocese of Chicago contends that the apparition reaffirms people’s faith, whether it is real or not. The archdiocese has no plans to try to authenticate the image. “These things don’t happen every day,” Jim Dwyer, a spokesman, said.

Previous apparitions in the United States have not always brought happy endings. A judge in Florida imprisoned Kyle Maskell, 18, for ten days last year after he used a slingshot to shatter windows in an office building that were deemed to reflect the Virgin Mary’s image.

In November an internet casino company paid $28,000 on eBay for a part-eaten, ten-year-old toasted cheese sandwich said to carry an image of the Virgin Mary. It wrapped the sandwich in plastic and sent it on a national tour.

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