I'd find my gas and water lines first, so I'd know where NOT to poke around with a shovel but, I don't know where you'd get that information, the utility or city hall. So many people here put in new services for their barns they must know how you to get that info. Good luck.
Definitely do NOT dig unless you have the utility inspector come out and make sure it's not buried gas lines or something. If it is an old septic tank and is not connected anymore, I think you can have it filled in instead of having to remove it entirely. They aren't considered "toxic" like an old oil tank.
I realize that I'm generalizing here, but as is often the case when I generalize, I don't care. ~ Dave Barry
If you suspect it's an old oil tank, it's definitely a "keep your mouth shut" scenario. It could get very, very expensive. Politically incorrect, I know. But political correctness most often comes at a steep price.
The reason the grass doesn't grow well over ours is that it's so close to the surface that the ground dries out very quickly. We also have several large rocks just under the surface and those areas turn brown at the same time.
“There are two ways to conquer and enslave a nation. One is by the sword. The other is by debt.”
The county engineer's office should have a plot map to your property, and an annotation of where things like oil tanks, septic tanks or other items are buried. It might just be that when they built the house they had leftover concrete on the truck, and dumped a bunch in the lawn (you can only keep it fresh on the truck for so long, or it gets too old to set properly), and it's just under the surface.
If you find out what it is, and ever resell you might have to disclose this, or remove it depending on state and local laws. It might be better to leave it a mystery, and plant sod over it if you decide to sell. Sometimes not knowing is better and cheaper.
You can call Miss Utility (or whatever it's called in your state, just google it) and they will come out at no cost and map your utility lines. Tell them you're considering putting in an ornamental fish pond.
I would dig carefully to see what I could find within a foot of the surface. If it turns out to be an oil fuel tank, just pack the dirt back down and keep the area spray painted green.
~Kryswyn~ Always look on the bright side of life, de doo, de doo de doo de doo
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