My Pool is Leaking

I am a relatively new pool owner. Moved into a home with an inground pool last year, it's approx 53'x23'. I have 2 skimmers and a main drain. My pool started to leak a few days ago. No water near hardware, leak is not visible. I've been working with my pool place, but am looking for further information. I suspect that the leak is in my main drain. My main drain when cover is removed has 2 holes, one on bottom, one on side. There does not appear to be any "valve" in either one. I was told that there is a hydrostat valve in my main but I can't see it. Can you please explain to me what the 2 holes are? I have plugged the hole going directly to the bottom, with my skimmer rubber plug for now, and pool looks like it may have stopped draining. Please help, what might be my problem and what am I looking at to fix issue.

Most drains are set up so they can be plumbed side-outlet or bottom-outlet. The unused hole is either plugged or used for a hydrostatic relief valve. I have no idea how your pool was plumbed.

A "hydrostatic valve" is a pop-up relief valve that allows water UNDER the pool to flow INTO the pool, when the pool is drained. The purpose is to prevent the pool from floating out of the ground when the ground water level is high -- but they often don't work, and they often do leak. If you don't know what these look like, you probably won't recognize them -- it would just look like that hole was plugged.

Thanks for the info. The problem is that neither hole had a plug. How do I determine which one is the relief and which one is the main?

If neither has a hole, the main drain may be plugged.

If the main drain is working, there will be suction flow, somewhere in the main drain when the pump is operating. Be CAREFUL, if you swim down: do NOT get your hand stuck on the hole, by valving 100% suction flow to the main drain!!