Posted: 08/08/2015 at 18:01
Really don't want to worry you Lily P but that doesn't look like an animal hole to me because there is no loose earth / scrapings outside. Having said that I have voles and moles which burrow up from underneath and leave neat little holes on the surface of the soil with no external scrapings. Those holes are, however, much shallower and smaller than yours.
It could be to do with your old tree. We have sunken areas of lawn where trees have been removed and the remains of the stump has rotted away - but these are wide shallow depressions - nothing like you have there.
Are you on limestone? If so, I would (carefully!) prod around the edges of the hole to see if the ground is stable. If any of it starts to give way I think you need an engineer there sharpish to see if there is any risk of a much larger hole opening up.
If you're not on limestone and / or the surrounding ground is stable I would do what you've done and monitor closely for further movement.
Really hope it's ok. What happened to the beech tree? It might be relevant if there is the chance of unstable ground there.