Digging up some treasures?

My father dug out a few more feet and got some dimension.
Foundation structure is 16'x36'.
Here's a few pictures of items I've unearthed between 4 to 6 inches into the ground.

If you have any knowledge on what this structure may have been, lets hear it.
Whats the reason for various colored rock/concrete in the foundation material?
What time era would you assume this structure was built?
What was the use of something of this size?
Why was it covered up?

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