Real estate is like a cake.  When someone owns land in fee simple, he owns the entire cake. 

When the mineral rights are retained, it is said that the minerals are "severed" from the surface.  The Seller, in effect, just sells the icing on the cake, while keeping all the layers of the cake underneath.  The Buyer just gets the icing on the top of the cake and owns the surface "from the grass roots up."

The pretty houses on the surface of the land are just like roses on the icing of the cake.