This is the simple filtration area! The primary job is to act as particulate filtration for the two pumps! The sand/clay layers are clearly visable and nothing was hidden back here. For my next project I will hide this area, although you can't see it from the front anyway. As this was still experimental, I didn't hide this or the colors developing in the substrate layers. I decided half way through construction, that I would build a huge one and I never really finished covering up some of the little ugly construction details!! Everything has been operational for several years now. Considering the pessimism that I encountered here when I first mentioned this a few years ago, I think that it has done rather well.