To lie in a position directly beneath.

To lie under or beneath.

“A stratum of clay underlies the surface gravel.”

To serve as a basis of; form the foundation of.

“a doctrine underlying a theory”

The entity from whose performance a derivative derives its value.

Lying underneath

“We dug down to the underlying rock.”

Basic or fundamental

“Points and straight lines are underlying elements of geometry.”

Implicit

“Many nursery rhymes have an underlying meaning.”