Gladon PP21EZQ 1/4" HappyBottom® Pre-Cut Pool Pad for 21' Round Pool - This HappyBottom® pool pad can add years of life to a pool and its liner. Made with special high density, tough Polyethylene foam, HappyBottom® pool pad provides a puncture resistant barrier that helps protect a swimming pool's liner. It also acts as a barrier to keep corrosion causing elements in the soil away from the metal components of your pool.