No, not the main drain. I have a 25yo gunnite pool. There's a plug in the deep end near the main drain, that I'm told is there to allow ground water in to the pool so the pool doesn't float, or the so back pressure doesn't otherwise cause problems. What's that called?
Whatever it is, mine dislodged this winter. Pool was half empty when I went to open it this year. Really pleasant when, after you see pool level drop on a rainy day, it goes up the sunny day after the rain.