Frillylily said:Also any large trees in the area that died in the past...
Lots of good answers and suggestions so far; I especially like the reminder to tie a string to yourself first.
Can't say for sure what your have but I would agree with Frillylily that an old tree might just be the answer. I have lived in my place for 14 years. A depression appeared and we called the property owner. He had seen this before and said it was from an old tree that had been cut down many years ago.
