[ApplegateRanch]

I wonder how far down I own the land my house is on.

That is a very complex question, which depends upon several factors. Generally speaking:

If it's something valuable under there, you own the surface only.

If it's an "environmental hazard" beneath, then you own as deep as it gets.

[Telepathic Intruder]

I’m reminded of baseball player Matt White finding that his property was sitting on something like a $billion worth of semi-precious rock. He’s managed to extract enough to retire on.

[null and void]

Or more briefly, you only own it if no one else wants it.

[Segovia]

How true! been there, done that.