8" pipe alongside pool, what is it? Not a skimmer?

[US Import]

Just moved to Virginia. Have had pools before, mainly in Europe, but this construction has me stumped. Not far from my skimmer is an 8" pipe that goes down about 10' deep (deeper than my pool) and holds dirty water; it doesn't seem to be connected to the pool so I'm wondering if it's something to do with the ground water levels? Right now it seems to only function as a breeding pool for frogs.... I popped a cup of chlorine granules in there just to clean it up and perhaps kill some of the wildlife in there.

Can anyone please advise me what this is, what it's for?

Screen Shot 2018-08-10 at 11.17.01 AM.png (254.86 KiB) Viewed 425 times

[Teapot]

Yes, I think you are correct, it's for ground water removal, if required a pump can be fitted

[Denniswiseman]

You really want to seal the top to prevent frogs and what ever falling in
When you want to pump it out you don't want if full of dead critturs