That first username I suggested doesn't really say much about stuff , he's mostly repeating other people's pictures and videos, namely from a couple other accounts: Here are 2 that do explanations..."ganjinh.2020" and "ganj.bastini_ " you just have to push the "translate" button below the posts... it's a little broken eglish but understandable... when they say plumb or similiar it means "plug". They show way more than anywhere I've seen, they are showing underground cap stones, and mine and crypt /vault entrances.
After following these guys, goto their main pages and click on the icon on middle right that looks like a profile of a person--- this brings up other suggestions, or similiar accounts to follow, there you will find another half dozen or so accounts to look at.....
Many have really good markers, etc... and give some opinions on them. There's also some fake stuff put out there on some to sell metal detectors or get "clicks" , but the underground cap stones are real...and so are the markers...
I'm going to spend the next several weeks reading up on the posts and wading thru the good stuff and the fake stuff and I would suggest others do the same... nobody from America is posting the underground stuff, but they are.
Edit: I'm finding the translated term Plump or plum means plugged container or room I think, although they are showing some plugged stuff which they are stating often times is a false spot.