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?

It may be an old return that was disconnected at some point. Usually the vacuum/cleaner port is under the skimmer. Is the opening threaded? You may be able to fit a plug in there so it stops becoming a breading ground for algae.

Yes it is threaded. It looks like I'll need to dig a bit for more information and to see if the port is plumbed anywhere or just capped off. Any idea why a return would be abandoned?

I have an extra return line in my pool and they put it in in case I wanted to use the type of cleaner that used the return. I use an electric cleaner. The return is plumbed to my pool house, but it's plugged on both sides. They may have done the same thing with your pool at that time when they built it.

The irritating part of his extra return, is the fact that they never plugged either side prior to filling the pool up. Underneath the line, I've got a pump house for my pond and by the time we discovered the lines to be "not plugged", my pond pump was completely submerged - SIGH... Now both ends are plugged. I could certainly use this line in the future for a cleaner or an additional return to my pool.