How far down does your property go?

If you own a plot of land in Fee Simple Absolute, it goes all the way to the center of the earth. Honestly. You also control a "reasonable" amount of airspace. There was a case where a farmer sued the Air Force and won. The Air Force was flying planes to low over his farm and his chickens were so scared that they would fly into the walls and kill themselves.

As for your question, as long as no other person or company has mineral rights to your property, you own all the dirt underneath your land all the way to the center of the earth.

The railroads own a substantial amount of property, and they will often only sell the "surface rights" to the land. This allows them to keep the mineral rights, so if oil or gold is discovered underground, the party with mineral rights gets to keep the property from the oil or gold.

You can ask for a survey of your property, or you can speak to an attorney to determine if you have fee simple absolute ownership, or merely a surface estate on the property.

By the way, technically, you don't "own" the land. You own an estate in the land. This developed from old England, when the king owned all the land, and he would allow the nobles to run an estate on the land, while still maintaining ownership of the land.