Old septic tank system...?

cistern?

picture?

If there are stairs or ladder leading downward, might be a bomb shelter. A few of these were built with much (highly unrealistic) encouragement from the government in the 1950's through the Cuban missle crisis. I remember neighbors where I grew up building one. They were sisters, both in their 80's at the time - no doubt there was a much better use for the considerable amount of money they spent.

Outhouses didn't have concrete sides or bases because their location had to be rotated from time to time as the pit filled up with waste. They are also located a respectable distance from a house for sanitary reasons.

If it didn't have that type of entrance, I'd have suggested a root cellar, but I think given the construction it was probably a water collection cistern. They are often in addition to a dug well in houses of that age. I have one outside my kitchen window collecting water from our spring. It was sufficient for the water needs of the time it was installed, when baths were taken weekly and automatic dishwashers and wash machines were a dream. We ended up installing a second cistern (collection tank) 1500 gallons to collect the run-off from the original cistern and have an electric pump to that one, but a pitcher pump in the kitchen drawing off the small cistern. Our small cistern is the same material as the well house at the spring head........brick. Most old ones will be brick or stone. But concrete ones certainly do exist.

Back on the family farm, the cistern was the original water source, and it collected rain run-off from the tin roof. I even have a cistern on a house I own in a near-by town to collected rain run-off. Built late twenties or early thirties. Many houses built before the turn of the 20th century had cisterns. My guess is cistern.

Not a pit privy (outhouse) but could be an abandoned septic tank. I have seen them that close.