Something that is covered and hidden by something else can be described as "the underlying ___". 

Physical examples of "underlying" things are:

• the wood in a house that's behind the walls ("the underlying wood")
• an old painting which has been hidden by dust and dirt ("the underlying painting")

The word "underlying" is also very common for talking about emotions and ideas:

the underlying problem

underlying emotions

the underlying cause of ___

an underlying assumption