What are the opposite words for surface?

Surface, the topmost layer of something, can have antonyms such as depth, interior, and bottom. Depth is the distance from the top to the innermost point or layer, antithetical to the surface that denotes the outermost layer. Interior is the inside of a thing, typifying the opposite of the surface that is the exterior. Bottom is the lowest part of an object, contrasting with the surface that is the highest part of it. All these antonyms demonstrate the opposite of surface and provide a range of choices to describe a physical entity in-depth while highlighting its various aspects and characteristics.

Antonym of the day

split down the middle
combine, join.