Ask anyone what you'd find inside their belly button, and they'd likely answer dirt, sweat, lint or a piercing.
What most of us don't realise is that we each have about 57 different types of bacteria living in our navel.
US researchers made the discovery after asking 66 men and women to swab their belly buttons with a sterile cotton bud. They then grew bacteria cultures using the samples, which the participants could view online.
"It was a fun way to reach out to the public and teach them about the ecology and evolution of everyday life," said study leader Dr Rob Dunn, an associate professor of biology at North Carolina State University.
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Belly buttons are less exposed and are washed less frequently than other areas of skin, so the bacteria are less disturbed.
Once the scientists sequenced the DNA from the samples, they found there were 2368 species of bacteria common to people's navels.
Most of the study participants' belly button bacteria were different, with only eight types common in 70 percent of the people involved –– most of the rest were unique to individuals.
"We got many more species of bacteria than we expected," Dunn said.
One of the common bacteria species was Staphylococco, which the researchers said helps fight bad germs, as well as Bacillus, which appears to protect the body from fungi.
Micrococcus was another common one found deep in the belly button, which appears to be able to survive without oxygen.
Dunn believes that if scientists can narrow down the most common belly button bacteria, they'll be able to better understand skin bacteria and how it works in conjunction with the immune system.
The study was published in the journal PLoS ONE.