Unknown plumbing connection

[eville]

I bought a house last year pool is original to the house about 1988.

I have an open connection that goes into the ground that I’d like to know what it is for.

1 is booster return for my “robot”
2 is my filter return
3 is an open connection

Don’t mind the mess and open connection I’m in the middle of installing a solar heat setup.

Thanks for any help!

 

[ajw22]

Use a shopvac and blow air into the pipe and see if you see bubbles coming out anyplace in the pool.