Synonyms of 'underside'
The base of an object such as a box or vase is the lower surface of it that touches the surface it rests on.
Plunge the base of the pan into a bowl of very cold water.
The bottom of an object is the flat surface at its lowest point. You can also refer to the inside or outside of this surface as the bottom.
the bottom of their shoes
Thesaurus for underside from the Collins English Thesaurus
The underside of something is the part of it which normally faces towards the ground.