Synonyms of underside
baseThe 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.
bottomThe 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
underneathUnderneath is also a noun.
• The liquid formed on the underneath of the top plate.
Thesaurus for underside from the Collins English Thesaurus