I had a small hole next to some plastic bed edging next to my house and thought it was a rodent burrow. It was about the size of a tennis ball so I filled it in. That was about a month ago. About a week ago my mower tire sank into it and I thought the dirt was settling and I'd have to fill it in again. Before I got around to it, it became the size of a basketball and had a large cavity inside. One side of it was sheer like it was done with a shovel. It took 4 40 lb bags of dirt to fill it in with a little mounded up on top. There is an old well about 10 feet away and a downspout about 2 feet from the hole but nothing has changed in years except we got city water a couple of years ago and discontinued using the well. Anybody got any ideas?