The navel is dark, moist, warm and rich with nutrients that are sufficient for the survival of microbes. there can be as many as 100 species of bacteria/microbes living down there in the navel of a single person with the average being 67 species. So what you smell is a combination of microbes, dead human cells, sweat, microbial wastes, digested fatty acids, butyric acid, acetic acid to name a few.
well, a little smell from the navel is normal, however, if it really stinks (like your shirt down there starts to smell like it too) and you can see pus or there is inflammation then there is a chance of infection. To prevent infection it is necessary to keep the navels dry and clean them every once in a while.
