What is this concrete slab design found in forest?

I found this odd concrete slab with these circles in it. It was buried lightly in the ground, there's also a wide random deep hole at the edge of a cliff followed by with what looks like a trench going down a long hill near it, it's been there a long time as well and there is a random, small rock wall that's almost buried. I found it near all this but it's so far into the woods on a mountain that it grabbed my interest. I want to know if anyone knows what this is i.e an old way of building a structure?

Picture link:

http://tinypic.com/view.php?pic=2zh4a45&s=6