We've been living in our current house for about 6 years. Every single year during this time, sometime around the beginning of September, a gigantic mushroom has sprouted at the exact same place (as far as we can tell) on our lawn. The mushroom grows from non-existant to full size in just a few days, and unless I do something about it, it stays until it's buried in a snowpile through the winter. For the first month or so, the mushroom seems to be a bit squishy (when tapped with my shoe), but later in the season it hardens up, to the point where it seems almost as stiff as wood.

I took a picture of it in 2002, but unfortunately neglected to add any sort of reference in before taking the shot. Let me say that it was just a bit bigger than a basketball.

This year, the mushroom has done something different -- in fact, it seems to be two slightly smaller mushrooms, growing side by side. I put a regular-sized brick in, for reference.

Do you have any unusual flora nearby?