I been working on my pool for that past few day now and uncovered the "main drain" cap, but I don't think it's a drain at all. The pool man in the past never used the "main drain" they always used a pump. Now it just looked like concrete that was closing tightly into a small hole. All there is on top is the top cover and ring thats it. nothing inside. I don't even know if there is a drain in my pool. But when my dogs hair hits the pool it flows to the main drain. I'm lost here..... Shold I cap it completely off of what. Here's a picture on it.