What is that gunk you find in your belly button called?
Lint.
I’ve always heard it referred to as belly button lint.
Well, there’s gunk and then there’s lint.
So which one do you have? I mean, we need to be clear here. 
Dr Karl’s 2002 survey on Belly Button Lint which won him an IgNobel Prize (honouring people whose scientific research “cannot or should not be reproduced”. )
The Perfect Master addressed the issue of belly button lint 37 years ago: Where Does Belly Button Lint Come From?
We had a short discussion of it in a thread, in which Cecil himself graced us with his presence: Belly Button Lint: Cecil Scooped: Aussie Wins Ig™Nobel Prize.
Now wait, I’m confused. Did Cecil scoop out his own belly button or did Dr. Karl scoop out the lint? 
It’s partly dead skin, which has trouble escaping crevices, and partly lint from clothing, which also has trouble escaping crevices.
I know that lint comes from clothing, but how? There are no sharp corners or rough edges on my bellybutton.
The belly button doesn’t scrape lint off your clothing. Lint is just going to come off all over your clothing anyway (Look at how much is in your dyer trap). But most of it is just going to slide off your body through gravity and flatness. The belly button collects things because it’s a crevice. If you have a wall with a hole in it, and you blow a bunch of dust around, most of the dust is going to be on the floor or in the hole, and very little on the wall…
Also (assuming you are male, and perhaps even if you are female) you probably have a noticeable line of body hair leading down to your navel. That helps to gather the fluff and guide it to its umbilical nesting place.
It doesn’t matter. It won’t come when you call it, anyway.
Dammit! Read the title as “Guns in Belly Button.”