If you've ever been pregnant, you may well have discovered a blackened mass called a navel stone protruding from your belly button. 

Normally hiding away in deep navels, as your belly expands they can be pushed towards the surface until a black or brown mass pokes its head out. They can be quite disconcerting, especially if you don't know what they are.

The good news (depending on whether you're pregnant or not) is that they aren't just limited to pregnancy, that's just a time when they can be spotted easily.

In one case report published in the British Medical Journal (BMJ) Case Reports, a 96-year-old woman showed up to hospital with a fever and loss of appetite that had lasted for a fortnight. Based on her symptoms and a urine sample, she was diagnosed with a urinary tract infection. However, during a physical examination of her abdomen, doctors found a hard, smooth mass just underneath her navel.

Though the mass didn't hurt even upon manipulation, and there was no soreness surrounding it, they noticed the tip of a navel stone peeking its head above the surface. A CT scan confirmed that there was a calcified mass in there, likely causing infection, and if left untreated could even lead to sepsis.

By soaking it in olive oil then squeezing it like a spot, they were able to remove the mass pretty easily.

So, what exactly are they, and how do they find their way in there? Well, it's essentially navel fluff (or tummy button fluff, or navel lint, or whatever it's called local to you). They are usually a mix of fluff or cloth from your clothes combined with congealed flakes of dead skin, fat, and protein.

If your hygiene levels aren't high, sebum – oily, waxy material made to protect your skin – collects with all the other debris (your skin and fluff) and, over a number of years, hardens within your belly button until it forms a tight, dense mass. It's essentially made from the same stuff as blackheads on your skin, though much larger and freakier to look at.

Largely, they can go unnoticed and won't cause any harm – other than a little nasty surprise when you find it in there. However, as in the above case, they can cause problems if left untreated. So if you have a deep belly button, it's best to make sure you practice good hygiene to make sure nothing is growing in there.