Follow-up on the mystery box:
Yes, it was a box of sorts. Well, it was 4 stone slabs forming the sides, and 2 slabs capping them. The top slabs weren't in fact that heavy, just totally stuck in the roots of the trees that had grown around them.
My friends and I hacked the roots with axes (great for the edges, this...), then we managed to lift the top slabs, and the sides immediately collapsed in - along with the sand around them.
It was basically an empty space with water at the bottom. Maybe 10" of empty space and 5" of water.
We tried to remove and sift sand from the bottom, to see if maybe there were some small objects mixed in with the sand, but no: there was a big fat nothing in there. After a while, we stopped as this - now fairly deep - hole isn't shored up and is collapsing inward as fast as we're digging it up.
There we have it. I still don't know what this whole constructions was really for.
One colleague of mine at work figured it might have been a hidden munition cache for the resistance during WW2 (this area saw heavy fighting during the war). But if it was, there wasn't a single cartridge in there - and probably never was, as no resistance fighter in their right mind would have dug up the ammunition, then rebuilt the entire cache empty.
It wasn't a well, as it's much too shallow. It wasn't a cistern. And if it was a simple natural cooler, why was it built atop a hidden cache in the ground?
Because one thing is clear: the cache at the bottom was deliberate and was definitely not meant to be found. I'm convinced of this.
It's a mystery alright, but one that will forever remain mysterious I guess. And it's boringly empty. So it fits right in the Dull Men's Club after all ๐
Now I have to backfill the hole, but not before I put the stone slabs back in with a bottle in the middle and a message for the next curious person who digs them up. It'll probably say "Hi there! I found nothing too!" or something similar ๐
Sorry everybody who got worked up! It was a nothingburger after all.