While digging up roots from vines, I hit concrete. What I hit is only a few inches down. It sits in front of a 3 foot tall retaining wall which is made of block and mortar. (The while runs about 35 feet.) This morning, I outlined the concrete slab which looks to be about 6-7 feet wide, and about ten feet long (coming away from the retaining wall). My instinct tells me it is a separate structure from the retaining wall. I haven't dug down because the wall leans in and any connection would have been broken long ago. And it wouldn't make much sense for it to be attached. Anyway, I did a rough outline. I did not remove all of the dirt from on top of it because 1) it would take a couple of hours, and 2) I didn't know what I would do with the dirt or concrete in the meantime. One thing I noticed is that on the right and the left side, there appears to be a slight lip. Maybe half-inch to an inch high. My questions is: What the heck is it? Someone who came to give an estimate on repairing/replacing the wall thought it was an old foundation for a shed. The previous owner had a shed built on top of about 9-10 blocks. This sits about five feet away from the slab. I was worried it may be an old septic tank. My guess is that a septic would not be placed in a backyard on a downward slope towards the house (which sits only about 25 feet away). I was advise that if it were a septic, it would likely have a cap which would be much smaller. Anyway, I thought I would shoot out for some advice on what you all might think it is and what the heck I can do with it.