Good Question...
I can't agree with Waldo tho... I mean, if you went "down" far enough, you'd come out on the other side of the world... Is that piece of land also yours???
Anyway, where I live, most of the houses ARE undermined and beleive me, the Gold doesn't belong to us
Also, we live near (about 10 km's away) OR Tambo Airport (AKA Johannesburg International, AKA Jan Smuts Airport) and have planes flying over all the time... in fact, the "Airspace" belongs to the Airport...
Interesting question tho... I wonder if there are any laws governing this???