Infinity pools actually have two levels: the main pool, and a basin that is strategically built beyond and below the infinity edge.  The main pool does have an edge, it is just hidden an inch or so below the top of the water. This means that most of the water is contained the same way as with any traditional pool. The extra inch of water that isn’t contained hides the wall and creates the illusion. The water that sits above the wall flows over the edge and into the basin below. It is then pumped back into the upper pool, creating a cycle where no water is wasted.