What is this, Dunmow airfield?
 

Was out walking around the old Dunmow airfield and came across this. It was positioned at the very beginning of the old runway 22 on the left hand edge, where the perimeter track enters the runway.
The top was removable and you could see down the round circular vertical shaft (about 3 inches in diameter) to about 2 foot where there was some water.
Any ideas?
https://cimg5.ibsrv.net/gimg/pprune....715b974c8.jpeg
https://cimg6.ibsrv.net/gimg/pprune....0290be8cb.jpeg

Judging by the dead grass around it I wouldn't drink the water.

It could well be a septic tank vent dating from the days before it was an airfield. Or a power/communications connection point?

I did wonder if it was anything to do with FIDO, because of its position,
but Dunmow isn't on any of the FIDO equipped lists I have seen

That thing looks in remarkably good condition for something that's been there for over 70 years. Suspect it's agricultural. The vegitation is fine on one side and dead on the other, suggesting something has leaked out to kill it and fairly recently.

Grass killed and top free to move? It's being maintained with weed killer and grease?

High pressure plumbing and you took the cap off? What if thousand-psi water had come out? 4 questions.

Had it been under high pressure I doubt that the OP could have removed the cap unless, when he set out for his walk, he'd thought to take specialised tools with him!

It looks like some kind of modern groundwater monitoring point to me. They're usually in a bigger diameter pipe, but the description sounds about right. Are they planning any developments nearby that might need a report?

It might help to know if the cap seals the top of the pipe, or if liquid/vapour can vent from under the box section on the side.

I wonder if there is anything buried there?

Land Securities (wretched developers) now own the old airfield and have applied to build 10,000 houses on it.
However the rust on the “cap” leads me to think it has been there for quite a few years

Don't blame the developers exclusively. Someone has taken their thirty pieces of silver to enable them to acquire the land to permit its exploitation.