A water stain.
What gets me is that no one ever seems to question why the Virgin Mary or Jesus or whoever would appear to the "faithful" in such stupid places. Do they think these symbolic people have nothing better to do but to manipulate toaster ovens to burn their image into cheese sandwiches, or make water stains under bridges, or whatnot? These are not holy places, nor are they the kinds of places (or objects) that are viewable to the world. I suppose you could make some argument for having to search for them, making it a pilgrimage of sorts...but I'll laugh, I'm warning you.
Interesting how these are the same kinds of people who DISBELIEVE the "face on Mars" (isn't it odd how, with all our Mars exploration, they conveniently decided not to check that out?). Whatever...I'm mostly just amazed at the amount of flowers and candles and things around this thing now...I can't wait for a good rainstorm to make it no longer look like this, because I'll find that funny, too.
Just goes to show: you can find any image you want, somewhere, if you just look hard enough. Personally? I agree with this person, who thinks it looks like a giant vagina.