A team of researchers dug into 60 different people’s belly buttons and turned up bacterial diversity and microbial mystery. All in all, they identified more than 2,000 species of bacteria as well as two species of archaea, the ancient predecessors of bacteria. Some of the species frequently occurred in multiple belly buttons, while others were confined to just a few participants. According to The Scientist:

Each volunteer harbored an average of 67 different types of bacteria, and the vast majority of the 2,188 species found were only present in six or fewer belly buttons. One of the volunteers, Dunn said, had not bathed in years, which yielded a belly button sample that not only had bacteria present, but two species of archaea, which were rare in the study.

The researchers are stumped over what causes this diversity and difference. Writing in Scientific American’s guest blog, the study’s lead author, Rob Dunn, ponders the mystery:

We began to more seriously wonder what explained the differences from one person to the next. We were finding hundreds and then thousands of species, many of which appear new to science. They included strange species, such as one species found on my body that appears to prefer to break down pesticides. One can imagine many factors that influence which bacteria are on your skin; whether you were born c-section or vaginally, gender, age, weight, whether you are an innie or an outie, whether you live in a city or the country, what climate you live in, whether or not you have a dog, and maybe even where you grew up or where your mother lived when she was pregnant with you.

The team is hoping that a new data set of more than 600 belly button samples taken from people all over North America may start to shed light on this mystery. And they haven’t even touched on the microbial forests lurking within our noses, eyebrows, toenails and armpits – at least not yet.

More from Smithsonian.com:

Want to Be Healthy? Manage Your Microbes Like a Wildlife Park

The Vast Influence of the Wee Microbe