I know this is an old thread, but I wanted ask a similar question. I have a similar pit, but there are no pipes going to/from it. It's just a concrete pit about 30" deep, 4' long, 3' wide. I want to just fill this in and cover it with concrete. But I'm a little worried that there's still 3" of water in it. So it seems water does leak into it from the outside walls. I'm at the top of a hill, and the house sits up and back from the garage. So it might be used to catch and pool underground water. I just wish I knew what it was really used for. Then I can decide if filling it in is a good/bad idea. If anyone can let me know. thanks