A sinkhole is a good possibility. Do you have heavy clay soil? The reason I ask this is that clay soil will maintain a cohesive cap over the underlying soluble rock cavitation. First thing to do is to stake off and rope the area, you do not want anybody walking over this area. Then get a rebar 5ft will do and drive it into somewhere within this perimeter. If it goes into the ground with little resistance compared with driving it into the ground outside the area, then a sinkhole is most likely. Now the advice that follows is based upon a USA point of view. Notify your home insurance carrier so they can have a claims adjuster come to your home and take a look at this. If the claims adjuster thinks that it might be sinkhole activity then they will have an engineering firm come out to your property to bore test holes for an engineering report. Your insurance company will then determine whether it is a sinkhole for which you would have coverage or another ground settlement issue for which you would have no insurance coverage should damage to your property occur. Yes I have had a personal experience with a sinkhole but on public property. On my way to my favorite limestone fishing stream I would walk by what I thought was a groundhog hole, only as the months went by the hole would be getting progressively larger. Then one day that little hole became 12ft wide and about 9 ft deep. I had been walking over the cap of a sinkhole all that time! P.S. I just checked the geology of your area. There is a high density of sinkholes in Hampshire due to the dissolution of underlying chalk beds. I would suggest that you get local advice to swiftly get on top of this situation -though not directly on top:)

Whoa! Very interesting! Thank you so much for your comment. I will take your advice for sure. Can't be too safe with these things.

Yes, I do believe we have clay and a lot of rocks on our land.

Many thanks!
Kind regards,
Lisa

We also have a couple of 'fairy rings' on our lawn: slightly longer grass than the rest, and occasionally with little mushrooms. So we gather that the rings are caused by fungus, but to get rid of it we'd have to dig down 3 feet! We're on alkaline/clay and just ignore them and mow over them - they don't give us any bother, and they're not too unsightly.

Never heard of a fairy ring before. It is a very interesting phenomena.

If its a fairy ring and not a sinkhole, its looking like a Grade 2, so it could be worse, at least its not a Grade 3 fairy ring. Different fungi cause Grade 1, 2 or 3 rings...

It currently looks more like a fairy ring than the start of a sinkhole, but in Hertfordshire, a large sinkhole opened up in Hemel Hempstead in the last year or so, so its not impossible. If your oak was dying and you didn't notice, the fungi responsible for this possible fairy ring are likely associated with that.

I think they can happen anywhere really, the one that opened up on the road out of our village some years ago was old mine workings , not mentioned on any maps.....
they do mine gypsum not far away but as a complete pleiosaur was dug many years ago from the village ..mining has been going on for many years........

Funny you should mention gypsum though, Pam - a high percentage of gypsum in bedrock is often associated with sinkholes because the gypsum wears away over years because of water movement/penetration.

Very interesting ,its amazing sometimes what you can learn I was thinking stone age barrow ;-)

Because of the gypsum mine(it goes for miles in all directions) the houses have to be left with pillars of rock under them because of subsidence, its interesting that the grass verge next door which used to be flat is now sloping towards the hedge, the house is set back behind an orchard......we have a septic tank and a 12 foot hole was dug in the field for it......solid yellow clay with big limestone rocks came out.....I guess we were once under the sea ......