base - a flat bottom on which something is intended to sit; "a tub should sit on its own base"

bilge - where the sides of the vessel curve in to form the bottom

heel - the bottom of a shoe or boot; the back part of a shoe or boot that touches the ground

sole - the underside of footwear or a golfclub

side, face - a surface forming part of the outside of an object; "he examined all sides of the crystal"; "dew dripped from the face of the leaf"

underbelly - lower side; "the underbellies of clouds"