It looks like someone had some soil (or something like that) in the large blue pot and had some runoff that hardened on the bottom. I tried to chip away the mess on the bottom but only got some of it up. That led to a different thought, the convex bottom makes it hard to get level items. What if we put 1/8 inch of resin in the bottom, it self levels and would not only take care of the curve, it would take care of the rock hard soil mess.
Good idea, or nonsense?