Do you have a septic system? If so, it could be an access lid.

Hard to tel wthout something in the picture to give it scale.

What is the diameter?

Inches?

Feet?

Thanks for the thoughts! The septic is out front of the house.

To give it scale, the tip of my size 10.5 shoe is in the picture and the grey thing laying on the side is the base of a bird bath.

It is roughly 12-15" in diameter. I tried picking it up, or pulling it up, with no luck. Granted, I did not try very hard in fear that it was connected to something.

"It is roughly 12-15" in diameter."

Sounds like a tank access port.

How old is the home? Did the house at one time have to have water trucked in? If so, it could be the lid to the old cistern.

It could be an abandoned septic tank.

You could always call the previous owner and ask her to be specific about what it is.

Thanks for the ideas. The previous owner only lived in it for 5 years, and the lady before that was the original owner. It was built in 1942.

If it is a cistern, how would I get access to it? How would I go about removing the cover to inspect beneath? Is it as simple as just digging around it and then lifting up?

it sure looks like an abandoned cistern opening. dig around the perimeter a few feet or so, that might give you more to go on. Most of the old cistern covers I have come across are square not circular though.

I wonder if there was an old outhouse here? Well opening? Hidden stash of cash?

Tap on that reddish cover, hollow below? sound thin? I would give it a big smack with a sledge and try to bust it open.

Maybe he fell into the hole?

"I would give it a big smack with a sledge and try to bust it open."
This suggestion must be from a member of Congress.
Then spend hours repairing it.
"Act in haste, repent at leisure".