We bought our house with the pool already here. At the bottom of this there is return coming out, which fills up the whole thing and then it drains into the tube which leads out an "eyeball" in the shallow end.
Is it for putting tablets in? That's the first thing that popped to mind...
It's dirty! I hadn't been able to get it open since we moved in. I happened to be spraying down everything and shot the hose blast into the eyeball as I was walking around the shallow end and the buildup of pressure popped the top off finally.
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