It is a spring-loaded valve under the main drain cover. If the water table rises above the pool floor, it lets groundwater into the pool so the shell is not pushed up out of the ground. On the coast, where the water table is high, this valve is important.
Over time its seal wears, and a valve that no longer closes lets pool water escape into the ground. This shows up as water loss that continues with the pump off and does not stop at any particular level. The leak is confirmed with a dye test at the drain, and the valve is replaced without draining the pool.
Still not sure it’s a leak?
Describe what your pool is doing, and we will tell you what it may be and what a visit would involve. If the water level is dropping fast, call us right away.