I bought an air chisel and was wacking away at the plaster and tile in the below-grade portion of my shower. I had assumed that tthey went below grade and did a layer of concrete around the below grade area. However as I was chipping away on the exterior facing wall, out came a mountain of ANTS! I quickly grabbed some killer/shield in the area (RIP ants), and chaged from the exterior wall to the wall adjacent to it. A few wacks later... another pocket of ants! It still seems like there is a solid surface behind the plaster, but somehow ants have gotten through and are living there.