Protuberate means to project or stick out, and its antonyms are "sink," "recede," "conceal," "flatten," and "submerge." Sink means to descend or lower, so the opposite of a protrusion is a sunk or recessed surface. Recede denotes a retreat or withdrawal, so the opposite of something that juts out is something that disappears or shrinks. Conceal means to cover or hide, so the opposite of a visible protuberance is a wholly or partially obscured object. Flatten implies making something level or smooth, so the opposite of a bump, lump, or hump is a even surface. Finally, submerge means to go underwater or sink below the surface, so the opposite of something sticking out is being fully immersed or engulfed.