On Monday, my daughter noticed a hole open up (about softball size) in our driveway, which is topped with limestone. That hole has since enlarged to volleyball size, and is larger beneath the surface. Seems that there is an old septic tank located there, and the lid has collapsed. (The city says that the sewer connection happened in 1948.) The tank needs to be collapsed and filled, and we have contacted a company about this. Kind of interesting, as this problem was not even remotely on our radar.
I have a picture, but can't really figure out how to share here.