(Reference: John Cook, Law and Life of Rome, Cornell University Press, c1967.)

Ownership of what lay beneath the surface of private land went with ownership of that land, including minerals.

Under the Principiate, mining properties became monopolized under the Emperor's treasury. Under "ancient custom," half of the yield of all mines belonged automatically to the treasury. The staker (or concessionaire) of a mining claim or "treasure trove" could have contractual rights to the other half of the yield for a 'time certain' by paying a license fee. He could also buy out the treasury's remaining half for a set sum. By purchasing the whole yield, the concessionaire acquired a real property right in the yield, valid for a 'time certain,' as long as he actually worked the mine. The concession could be withdrawn if he failed to work the mine.