A couple of years ago a small depression (2 ft wide/1 ft deep) appreared in the backyard. I filled w/ topsoil and planted new grass. The following year, the sinkhole got bigger and wider. I filled with more topsoil and planted more grass. The hole keeps getting deeper and wider (now 4 ft wide/2 ft deep). It's still 15 ft from the house, but it's close to a very large tree in the yard. Anyone have any advice?