I was sort of joking about the grave.
but
when I do a pumpkin trench, I dig down a foot or two and fill with rotting kitchen waste. I get rodent tunnels that look identical to those in the piccy. They only do it to get to the food underneth, they dont tunnel into an ant nests.
The clay under epping is yellow london clay. It looks like the clay from a few feet down has been deposited up on top. Could moles do that?
Thinking about it if you had a job of having to dig a hole large enough to deposit maybe a ten stone carcass, and you had a to do it in the dark in a hurry , you might do a shoddy job esspecially if you weren't used to doing that type of job. If you were paid to a proper job and were suitably qualified
, you might take more time with an axe and saw, and something creative like an acid bath, take away kebabs, a hungry pig, or a boat trip to isle of sheppy. Hopefully no-one reading has any expertise on body disposal methods, i just know quite few murders victims have been found by mushroom hunters and dog walkers looking for a dog walking off a path.
It is probably something natural, but personally if there is chance it isn't i would get someone to look at.