We have an in ground pool and the pump and sand filter is located in the basement. We noticed a leak a little while back coming from the sand filter. It was winterized by a professional, at least that's what he calls himself. Jets have been plugged off. I was able to break the ice off from it today and I noticed the lid off of the main drain cover had came off. We called a pool specialty place and they said it was impossible for the main drain to be leaking in the basement. Nobody is able to dive down until the weather warms. Are there any ideas about what it could be? I appreciate any help.