My guess, and it is just a guess… it’s a storm water control system, possibly cut into the sewer line.

I’ve seen older houses where someone cut into the basmeny floor directly into a sewer line to put in a basement drain.

Based on you saying water was in there when it was raining, someone might have cut into the storm water mitigation line for a basement drain. Then someone tiled the floor and put a vent over the hole because that’s what they found that fits.

With older houses, a lot of times, the storm water system was tied directly into the main sewer. If that’s what this is, I’d be concerned about sewer gasses coming up from it.

Again, with only seeing one picture, it’s just a guess.