" " It's a jungle inside your belly button. John James/Getty Images

Pop quiz: How many of you think the reason you have an "innie" or "outie" belly button has something to do with the way the obstetrician cut and tied off your umbilical cord when you were born? Come on, be honest! The persistence of the innie/outie myth is "Exhibit A" that we don't know jack about our belly buttons.

Your belly button is the leftover remnant of what was once the umbilical cord, the rope-like connection between you and your mother that supplied all of your nutrients and oxygen when you were in the womb. When you were born, the doctor (or maybe even your shaky-handed father) cut the umbilical cord a couple of inches away from your belly and clamped off the remaining section.

There's no tying involved. Once it's clamped, the small section of umbilical cord dries up and falls off in about a week. What's left is the umbilicus — or belly button. And the shape and size of the belly button depends entirely on the way your tummy heals after the cord falls off. If you have an outie, it's likely due a mild umbilical hernia or slight infection of the site. Roughly 90 percent of people have innies.

Here are five more fascinating bits of navel news.



1. It's a Jungle in There

According to a delightfully odd scientific initiative called the Belly Button Diversity project, belly buttons are home to a startling diversity of bacteria. The fun started back in 2011 at North Carolina State University, when a team of young researchers got the idea to explore the microbiome of the belly button. Would the bacteria colonies in the navel be different than those found on the rest of the body?

Using RNA sequencing, the researchers identified 2,368 different species of bacteria living in the navels of 60 volunteers. For reference, there are half as many species of birds or ants in North America.

Although eight species of bacteria dominated the belly button microbiome, accounting for 45 percent of the total population, there was still incredible diversity among individuals. For example, no single bacteria were found in every belly button, and 2,188 of the species were only found in 10 percent of sampled belly buttons.

One dude (and we're assuming it was a dude) not only carried loads of bacteria in his belly button, but also two species of archaea, "a domain of life often found in extreme environments and not previously reported from human skin," wrote the researchers, adding that the individual "self-reported not having showered or bathed for several years."



2. Your Belly Button Is Connected to Your Liver

The question, "What's behind your belly button?" sounds like a Zen riddle. Nothing, right? But just like your belly button is a leftover remnant of the external umbilical cord, there are also internal vestiges of the prenatal connection with your mom.

Remember that the purpose of the umbilical cord is basically to circulate the mother's nutrient- and oxygen-rich blood in and out of the growing fetus. To do this, the umbilical cord contains two types of lifelines: an umbilical vein that delivers blood to the baby; and two umbilical arteries that carry waste and carbon dioxide out of the baby and back into the mother, who can dispose of them.

Inside the growing fetus, those umbilical veins and arteries connect to the circulatory system, the liver and the bladder. When the baby is born, takes its first lungful of air, and the umbilical cord is cut, the internal sections of the umbilical veins and arteries also dry up and harden into a type of ligament. But those ligaments are still attached to the inside of the belly button.

One of the ligaments connects and bisects the liver. Another stretches down into the pelvis where portions of it may still function as part of the circulatory system near the bladder. In some babies, the vestigial artery running from the bladder to the navel doesn't close entirely and urine leaks out of the belly button. A simple surgery can close it back up.



3. Hairy Bellies are Lint Magnets

Belly button lint, like earwax and toe jam, is one of the great unspoken mysteries of the human body. Back in 2002, an Australian scientist and radio personality named Karl Kruszelnicki, aka Dr. Karl, decided to find out exactly where belly button lint comes from and why some people are afflicted (blessed?) with it and others aren't.

Dr. Karl conducted an online survey where he asked people whether they had belly button lint, what color it was, and details about their skin and body. From 4,799 replies to the survey, Dr. Karl was able to make some correlations. Moderately overweight older men with hairy tummies were the most likely to have belly button lint.

The hair on the stomach traps bits of cotton fiber from clothing, and the movements of the shirt over the hairs funnels the fibers down with gravitational-life force toward the black hole that is the navel. When volunteers shaved the hair around their belly buttons, 40 percent noticed the lint trap disappeared.

Dr. Karl was the proud recipient of the 2002 Ig Nobel Prize for his research, an award handed out annually for "achievements that first make people laugh, then make them think."



4. Some People Don't Have Belly Buttons

All placental mammals have belly buttons. That includes cats and dogs and beluga whales, although it's often harder to see them on animals. Yet oddly enough, not all humans have belly buttons.

Supermodel Karolina Kurkova is one of them. You'll almost never see a picture of Kurkova's navel-free tummy because magazine editors photoshop in a replacement belly button so people don't think she's an alien. But screenshots from midriff-baring runway shows reveal a slight indentation where the normal innie or outie should be.

Nobody is born without an umbilical cord, so all of us should have belly buttons, right? But some babies are born with issues like umbilical hernias or a more serious condition called gastroschisis, where a baby's intestines stick out from a weak point in the abdominal wall. Those conditions can easily be corrected surgically, but the resulting scar doesn't look much like a belly button.

People who undergo tummy tuck surgery may also remove the band of flesh where the belly button resides. Some opt to have a new one created in its place, while others see it as an excellent opportunity to ditch their lint problem once and for all.



5. Navels are a Surgeon's Best Friend

If the eyes are the windows to the soul, then the belly button is the window to the gallbladder. In the field of minimally invasive surgery, more surgeons are performing major procedures without serious scarring by going through the belly button.

Laparoscopic surgery is a type of minimally invasive surgical procedure where surgeons make a small incision in the navel and insert a laparoscope, a telescope-like tool with a light on the end that enables doctors to see what's going on inside the gut without opening up a large incision. In a typical laparoscopic procedure, one or more additional small incisions are made to cut out and remove the target tissue.

But now there's a growing interest in single-port laparoscopic surgery, where both the laparoscope and flexible surgical instruments are inserted through a special port plugged into a single incision in the belly button. Not only is scarring hardly visible, but a single incision shortens recovery time and lowers the risk of infection.