When the man discovered something dark in his belly button, he decided to get to the bottom of it — with a pair of tweezers. What emerged from the depths had obviously been stuck there for quite some time.
A navel stone, also know as an umbolith, consists of dirt, fibers, and hair particles which stick together. If left for a long time, this lovely clump can become as hard as a rock. The best means of prevention? Don't neglect your belly button when bathing!