Here are a few possibilities:
* For a geographical surface: You could be talking about land, rock, or ground.
* For a physical object: You might call it the body, frame, base, foundation, or shell.
* For a mathematical surface: In geometry, the solid part of a surface might be considered its interior or volume.
Could you give me more context about the surface you're referring to? This will help me give you a more accurate answer!
