What occupied this spot in my closet?

It’s my Ikea shoe holder. I took it out because the closet is being painted (and wallpapered) so I needed to strip down to basics.
Look what has been accumulating from my shoes over the last decade.
This wasn’t me being lazy; the shoe holder is affixed to the wall. So the only way to clean under it is to remove it.
Anyway, super gross, but it cleaned up fairly easily.