Uhh... maybe an old septic tank? Cover plate for old well? Have you dug around yet?

What's the lay of the land?

Are you planning to build in an old war zone, perhaps?

Its orthern Europe, probably was has been there, but there are other houses around and as pointed in schematics there are pipes for rain water drainage, those are just 50mm in diameter and not metalic. There could be a domolite, bet that would be I guess 2-3meter deep, i have heard that detectors could pick that up.

Time to investigate or have an expert do it. Contact your building department for anything like this already discovered.

Ye Olde World is littered with the debris of thousands of years of ravaging warfare. Consider underground bunkers, which are still turning up in the Netherlands. Or a private modern fallout shelter fallen into disuse. Sweden has 65,000 shelters and continues building new ones.

Let us know what you find. (Unless it's prosaic.)

Yes, the tractor will be there on Monday and then we shall see (starting to build fundament). As I can remember, the detector even gave signals when it was raised against horizon in that area, but no electric line comes near that place.

Following

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Following. Hope no kaboom.

OP has not responded in over a year; obviously an alien abdication. It was a trap. I love old posts.

Hey, hey! So, tractor worked all day for fundament, but I saw nothing there.. I hoped to find some meteor, but nothing. Now, I have not tested anything. Fundament is build and aerated block walls are growing now day by day.

Well, I think this could come out from your metal detectors. I mean both of yours could be broken, you have to check them or to use a new one to see what's going on under your house. But I am pretty much sure there is nothing to be. I would recommend you to buy a new one, and you will not have those problems any more. You can find a couple of great metal detectors on this site https://metaldetectorlink.com/best-metal-detector-for-beginners/ I bought one last year and to be honest, this is the best one I've ever used. More than that, you can a buy a new one for your son if you want him to start to practice to search for metals

Why would it explode?

We arent diging anymore.