A drained pool looks harmless until you realize it can become a boat. We’re talking about pool pop-ups, the real-world risk behind the phrase “the pool can pop out of the ground,” and the simple physics that make it possible when groundwater rises under an empty shell. We walk through what actually holds an inground plaster or concrete pool in place, including the massive weight of thousands of gallons of water, and what changes the moment you drain it for an acid wash or to lower high cyanu...