The bolts seem to indicate something was once attached there: hot tub or gazebo seem most likely. There's only one pipe, so that seems more likely to be electrical rather than plumbing, and therefore a gazebo rather than a hot tub. Have you tried shining a strong flashlight down the pipe, and if so, what do you see? Anything that might be wiring? The "clog" a foot down is probably simply a bend in the pipe. Just to rule some things out: Are you on sewer or septic? Are you on municipal water, or do you have your own well? Did you ask the realtor what it is? Have you asked the neighbors? (Not simply the neighbors on either side of your property, but people in the neighborhood who might have lived there for 15 years or more, particularly if they were close friends of the people who lived in your house.) If you're adjacent to any of the country club facilities, you might ask them also. But in the meantime, I vote for the ghost of a long-gone gazebo.

The honeybees here are in love with Mignonette. And we are having our fair share of sweat bees and bumblebees too. Apparently, they are doing what they are supposed to do, because I've got plenty of little tomatoes. And I expect to see more, as the Agastache is getting ready to bloom. It's definitely a welcome sight, seeing them all over the place this year.

The easiest way to ID weedy grasses is from the inflorescence, the seed producing stalk. Compare your weed to Dallisgrass, you could also try asking on the grasses forum or Name That Plant. Good luck!

I live in Appalachian Ohio in mine country, and in some parts of our county you need a rider for mines on your home owner's policy. I have to have one at a property I own in town since that section was built over old mines. I also have an old coal bank on my rural property. It was mined privately by an individual, by pick and I'd expect it not to have conventional looking excavating, either. There may be no rhyme or reason why this wasn't just filled in and a piece of wood thrown over it and left to rot. It could be anything, actually including an old dug well or spring or rain water cistern where the bricks had been removed and used for landscaping or something, lol. That's almost the exact measurements for my spring cache basin near the house.