By now you've probably heard about the image of Mary on the wall of a highway underpass in Chicago (picture below). I would accept the most obvious explanation that the image was formed by natural causes--water and salt runoff from the highway above. While I don't see it as a miraculous image, it's still very interesting because of the way it can speak to us of God.
The people who saw the image recognized something in it that looked like the Blessed Virgin. And they've responded with faith. A sort of makeshift shrine has popped up at the site. So even if the image itself is not miraculous, it's evoked a response of faith. In today's secularized society, that in itself is miraculous in some way. It's a reminder that everything in the world can speak to us of God if we see it through eyes of faith. Just the fact that this image appeared right around the beginning of May, Mary's month, is also significant.
There's an excellent book called Encountering Mary by Sandra Zimdars-Swartz. In it she studies several Marian apparitions, but from a particular point of view. She looks at how the seers' experience of their visions was shaped by the response from the people they shared their story with. At La Salette, for example, Melanie and Maximin said they saw a lady. They didn't identify her as Mary, but the people in the town immediately came to that conclusion. That in turn influenced the way the children experienced their further visions. Even when a vision is authentic, the person who receives it passes that experience through their own mind and draws conclusions about it. They make deductions from what they have seen.
What do you think about the wall in Chicago?