Huge problem! I have a leak in my inground pool!

I noticed the water level being lower than usual so i topped off the pool. Then when i turned the hose off, i noticed the level come back down again within a day or two. The level is now about 4 inches below the skimmer openings so i think i can eliminate the skimmer as the source of the leak.

Its leakingnwith or without the filter running so i dont think its the plumbing lines from the skimmer to the filter.

The only other thing it could be is the main drain, right? How can i test this? What should be my next step?

Have you been running your pump? It could be leaking somwhere in the return lines. Or maybe a failure in the plaster?

Well I closed the pool as normal in the fall and when i removed the cover the water has drained all the way to the deep end. I'm pretty sure its the main drain now.

Got a pressure test scheduled for this week. I'm really hoping its around the drain and not in the pipes. The pool guy is recommending that I plug the main drain if its in the pipe. I really dont want to do that because i know that my pool depends on the main drain to remove a lot of the debris at the bottom of the pool.