Some people believe it was just a stain on the wall. To others it was a miraculous apparition of the Virgin Mary.
It was dubbed "Our Lady of the Underpass" for its location on Fullerton Avenue under the Kennedy Expressway.
Since it was first noticed in 2005, the apparition has been defaced and painted over. The crowds of pilgrims have faded. But fresh flowers and recently lit candles still adorn the site, surrounding a framed photograph of the original image.
Those signs of continuing devotion made an impression on Bishop James A. Wilkowski, who leads the Evangelical Catholic Church, a denomination independent of the Roman Catholic Church and claims 5,000 members.
"This has not faded away, places like this are a testament to peoples' faith," he said on a visit to the underpass Sunday.
"You can't take away faith no matter how hard you try. You can dismiss it, you can belittle it, but faith is far more powerful than a paint-over job," Wilkowski said.
Pointing at the many messages to the Blessed Virgin scrawled on the concrete wall, Wilkowski said they demonstrate "the hunger" for something spiritual in the lives of many.
"Maybe this was not like Lourdes or Fatima," he said, speaking of two Virgin Mary apparitions officially recognized by the Roman Catholic Church. "But something happened here in Chicago that touched people and caused a response."