I was thinking along the lines of the solids portion of an effluent tank as well. There don't appear to be any incoming/outgoing pipes though, and it's pretty small to be handling sewage.
If the only access is from above (seeing as it wasn't capped off with another slab) I'm leaning towards a freshwater cistern. If you do a google image search for "old cistern" you'll see a few that somewhat resemble what you've uncovered.
-Mcoy
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