Demo started yesterday of removing the fiberglass overlay from the IG Gunite. I got back and there's a 1'x1'x1' hole approximately 4-6' up from the main drain. What is this? This is my first pool remodel and the PB hasn't told me what the steps will be for demo. Is this a "vent" for hydrostatic pressure? We are getting rid of the fiberglass overlay, redoing the coping and tile and having Pebble Sheen Ocean Blue. Any advice or insight is GREATLY appreciated.