Thanks for your responses!
We have yet to remove the plywood and get a really good look inside, but as far as I can tell it is jus a perimeter wall and there is gravel at the bottom with an occasional cat living down there.
It might be hard to tell in the pictures, but its really, really big and pretty close to the house. A
pond of that size would just be too much and too close for comfort. It's maybe 5 feet away from the door, would hate to trip and drown coming out of the house...lol.
Down the line we do want a green house and an aquaponics system, so filling most of it will be the option for us.
I like the idea of a mound on the perimeter so we won't need to go to the effort of knocking down the wall, but near the porch I guess we could rent something that we could use cut it level to the ground, then use the pieces as stepping stones or something like that near the shop out back. Same with the wide path with the pillars....has to go.
Would the leaves from the black walnut be a problem for the pond? I'm sure the roots wont because there will be a liner.