Since the only outlets/inlets are at the top it was most likely some underground storage for fluids such as petrol, what to me is more puzzling is how it got where it now is🤔??
Did you see any aliens lurking about in the woods as they could have crash landed and that's all that remains of their once proud intergalactic craft. I can't decide which is the more likely 😂

JohnH: no aliens but just before spotting this I did come across a pure white pheasant, which was a first for me.

I saw Ringo Starr heading your way with a pot of yellow paint. No idea why.

dennisgrant: 😊

How could it be a pure white pheasant when the hairdressers don't open till this coming week, perhaps it got its feathers dyed in some illegal salon, such as the big tank!!! 😂😂

Clearly a "think tank" as so many of us have been thinking about it.

top_spot (john): So long as it is not a septic tank that I was clambering over while trying to get a look inside 😮

If it was, at least you didn't fall in 😂

It's the new stealth tank, leaves no tracks 😊

Blue John: Ahhh! a stealth tank disguised as a submarine 🤔 mmm!