Mystery

[Mike Allen]

Perhaps the american moles are bigger

 

[Tetters]

Yep - I daresay they are

 

[DirtMechanic]

A method for checking or rather comparing water content of different areas of your soil would be to compare a relatively undisturbed volume of soil to the same volume of water. The weight of water would be the standard and would be necessary to determine and compare the specific gravity of the soil sample to that of water. A dried sample could or would be compared as well. I would think a very fine clay capable of swelling would have to be imported to that area, but it is strangely not out of the question. Yazoo clays are exported from Mississippi to seal pond bottoms and they swell enough to make humps in the interstate roadways of MS.

 

[SeniorCitizen]

A soil sample from the area in question and samples of the surrounding area may prove to be interesting.

 

[Oliver Buckle]

I had an underground ant's nest. As they excavated they dumped tiny grains of soil on the surface which were not visible in the grass but gradually made a lump. It would have to be a big nest to do that though, still everything is bigger in America, right? If it were me I would be tempted to lift the turf, dig it out a bit, and put the turf back level. It would make mowing easier and you would get a look at the top few inches. Knowing my luck though it would then probably sink again. Sorry I don't have any really good suggestions, but please keep us posted, this is fascinating.

 

[Marck]

Always curious about mysteries. Has any excavation attempted or other update? A photo would be interesting.
The rock and ant hill ideas are intriguing. I wonder if an exceptional fungal mycelium colony could create such a mound?