[backhoe]

It could be any of the above... the old mechanic who taught me much of what I know had a specially set aside corner in his yard where he poured & then buried motor oil. A high water table would force something like that to the surface.

BTW, check your deed for any strange "mineral rights" clauses-- a house I bought years ago for The Front Porch ( one of my stores ) had a specific clause which meant that any oil, gas, or valuable minerals were not mine, but reverted to the former owner. Didn't mean a damn thing in practice, since no such deposits existed, but the seller was covering all his bases.

[varon]

.........the old mechanic who taught me much of what I know had a specially set aside corner in his yard where he poured & then buried motor oil............

I was thinking along the same lines that perhaps a neighbor has been changing oil in his yard and dumping the old oil which has eventually seeped into other properties.