Ask the Pool Guy: Max Asks: What is under my swimming pool liner? {Pool Kids} Ask the Pool Guy April 20, 2014 Posted in Ask the Pool Guy, Ask the Pool Guy - Bonus! Share: TwitterFacebookLinkedin Max Asks: What is under my swimming pool liner? Mark explains the types of bottoms commonly found underneath swimming pool liners. Sand and Cement/ Vermiculite/ and Sand. Author: Ask the Pool Guy