Well I'm an Old Fart that thinks it is the Filler Cap for an Underground oil Tank! Im going to make a guess that it was filled with Water or a slurry of Water and Dirt to conceal its existence !
Cheaper than digging it up but not the correct method that would be demanded These Days!
I heard a story about one of these being buried in My Community a few Years ago when a School Building was beIng demolished and the Land redeveloped for Housing and a small shopping Center
You could be in for a serious discussion from Your local building inspector?
