No two are exactly alike — even for identical twins.
The belly button is the scar on the abdomen that is caused when the umbilical cord, through which the mammalian fetus receives nourishment, is removed. Unlike other placental mammals, the human umbilicus is quite conspicuous. In humans, the navel is the center of gravity of the body.
Omphalos, an ancient religious stone artifact, means “navel” in Greek. The legend goes that Zeus had two eagles fly across the world to meet at its center, or omphalos. The mythologies of many cultures consider the navel a primary component of creation myths.
When the scar appears as a depression, it is referred to as an innie. When it appears as a protrusion, the opposite colloquialism applies: outie. Some who are unhappy with the appearance of their navel opt for umbilicoplasty, or belly button surgery.
The navel was recently in the news. A study found that runners of West-African origin typically have a higher center of gravity than their competitors of European origin. In other words, their belly buttons give them an advantage on the track. Europeans have an advantage in swimming competitions because they typically have longer torsos and lower belly buttons. No mention was made regarding the effect of innies versus outies on performance. Do you think the center of the world is more likely in an innie or outie?