if you own a parcel of land, you technically own from the center of the earth into the heavens above you.


in legal theory and I quote "a tract of land consists not only of the portion on the surface of the earth, but is an inverted pyramid having its tip, or apex, at the center of the earth, extending outward though the surface of the earth at the boundary lines of the tract, and continuing on upwards to the heavens"

this definition is from Kratovil and Werner, Real estate Law, 8th edition. 

This hold true except for things such as when Congress passed the Air Commerce Act of 1926, basically allowing " a public right to freedom of transit in air commerce through the navigable air space of the United States"...basically to keep land owners from either charging use fees or restricting air transit from traveling over their property, but technically you own the air above your land and everything under it.

the owner of the entrance only has the legal right to allow access from the entrance to their property boundary unless otherwise granted from the adjacent landowner...technically an adjacent landowner could put up a fence and gate at the property line undergournd in the cave and prevent you guys from going under their property...not likely for anyone to go to that kind of trouble though.

I've done appraisals where someone owned a small homesite on top of a mountain, and thousands of feet below the site, the Army Corp of Engineers was tunneling through the mountain to divert a stream channel, and it was our job to determine how much the landowner thousands of feet above them on the mountain was to be paid for the portion tunnled under their land...they technically owned to the center of the earth...so the army had to pay them even though there was no direct damage or even any visual damage to the property.