The term "bulge out" refers to something that protrudes from a surface, indicating a bulging or swollen appearance. Its antonyms, on the other hand, suggest that something is sunken or recessed. Antonyms for bulge out include words such as sink, retract, cave in, hollow, and concave. These words describe items that are not bulging or bulging outward from a surface, but rather sinking or falling inward. For instance, a sinkhole can be considered an antonym of bulge out. The sinkhole forms due to the collapse of surface layers, causing a depression in the ground instead of a bulging outward.