Belly buttons are scars from the umbilical cord joining point during gestation (ie baby development) in the womb. We all have these scars and after birth we all have “outie” belly buttons until we grow further when they turn inwards. This “innie” inwards style is what the vast majority has.
One interesting of belly button biology is that what lives there. We all have lots of microscopic organisms, like harmless bacteria, living in us and on us – belly button included. And so scientists have start surveying what microbes live in our belly buttons. They used DNA sequencing of our belly button “jungle” in 60 people. They found 67 microbial types living in each person’s belly button, and that each person set of belly button microbes was quite unique! They also examined person who did not shower often, and found that they had a more diverse set of microbes living in their belly button! See the links for more
Did you know that, to prevent scarring, some surgical procedures are carried out through the belly button? These are called Laparoscopic surgeries and can be performed to remove the gall bladder or appendix. There are even robotic systems, such as the Spider Surgical System (http://www.transenterix.com/how-does-spider-work.php) which can perform the surgeries.
The instrument goes in through a small incision in your belly button, and when inside opens up like a cross between an umbrella and a Swiss army knife! It has different arms with scissors, clamps and other surgical tools as well as a camera so the surgeon can see what is happening. Once the surgery is finished it closes back up and is pulled, along with the removed tissue, out through the belly button.