There was a lot of dog hair and a lot of dirt. It looks like something spilled on the floor and oozed underneath a bit. When did that happen? I'm sure one of the kids knows.
Now that it's clean, it looks better than the rest of the kitchen floor. That little pice of wood is the shim that keeps our refrigerator level, in case you were wondering.
While the fridge was out, I vacuumed the coils and everything else that I could reach with the vacuum wand. It was quite dusty. I know the fridge will run better and use less electricity now that it's been cleaned.