While not actually "under construction" I have a construction/design question. I've recently moved into a house with an older pool which I am trying to figure out. I've marked up the attached photo of my pool. I have two intakes (skimmer and drain) and two jet discharges. Opposite the skimmer in the deep end is a "mystery" port. It currently is neither intake or discharge. It is just a hole that accumulates algae. I haven't yet dug up the yard to see if there is plumbing connected to it. I've tried a garden hose to clean it out, but the hose only fits a few feet in. Does anyone have any idea what this port is? Is it a dummy that was never connected; a possible vacuum port so the skimmer can also be used during vacuuming? I know I'll eventually have to dig up the yard side to see if anything is connected or if it is just stubbed out and capped. I'm just curious what it is! Any ideas?