as a preposition, 'underneath' indicates a position directly below or beneath something else.
It is used to show the relationship of a subject or noun to another entity in terms of position. It often conveys a sense of being covered or obscured by what's above.
The cat is sleeping underneath the table.
You'll find the spare key underneath the doormat.
The roots of the tree spread out underneath the ground.
'Underneath' and 'under' can often be used interchangeably, but 'underneath' can sometimes emphasize a deeper or more covered position. For example, while 'under the bed' might mean just below the bed, 'underneath the bed' might imply a position further in, perhaps in the center of the space below the bed.
as an adverb, 'underneath' describes a position or location that is directly below or beneath something else.
It can describe where something is located or the direction in which something is moving.
He looked under the car and saw the cat sitting underneath.
The foundation was unstable, and the house collapsed underneath.
She wore a jacket with a bright shirt underneath.
when using 'underneath' as an adverb, it's not followed by an object, unlike its use as a preposition. Ensure that the context makes it clear that you're describing a position or direction.