More than likely they have no mineral rights.
In California, I believe nobody gets mineral rights, and I'm sure that's similar most places today.
That would be incorrect. Unless you or a previous owner has specifically relinquished any of the rights to your property, you still possess all of them, mineral rights included.
In California, "real estate" is the same as "real property" and includes land, fixtures to land, anything incidental or appurtenant to land, and anything immovable by law.
Incidental rights (as used above), includes air rights, surface rights, subsurface rights, mineral rights, water rights, riparian rights, littoral rights, and underground water rights.
Few people take the time to actually get a copy of their deed and covenants and read them through. It's not that hard to do and I would recommend you do so. If you (TGF) need help doing this in Santa Clara County, Freepmail me.
--Boot Hill