Doesn’t the petroleum industry have CATscan techniques to answer this sort of question?
Shhh! I am sure the History Channel has a couple brothers who can spend quite a few TV seasons trying to figure out whether the Knights Templar, or the Mayans dug the tomb and there hid the Holy Grail, or the Ark of the Covenant. If they go straight to solving the mystery it will kill years of advertising.
Tomography. There are several different types but I’d think one could get a measurement of density in several directions to indicate voids or less or more dense ground.