Navel

A navel (commonly called a belly button) is a protrusion or an intrusion found on the lower abdomen of mammals after they are born. It is where the umbilical cord is connected to the being during pregnancy, after which, when the umbilical cord is cut following birth, it dries up and falls off from the being, leaving behind the navel.

It is not likely that navels appear on any Smurfs in mainstream Smurfs media, given that male Smurfs are born through non-physical means and female Smurfs are generally magical creations. However, in the EMPATH: The Luckiest Smurf story series, navels do appear on naturally-born Smurfs due to their physical reproduction.