This HS502 Premium Hard-Side Waterbed Mattress is part of our Hydro-Support 500 Series featuring Strobel's Patented Double-Wall Leak Proof System for added peace of mind, deep molded feathertop pressure reduction surface for greatest conforming comfort, heater compatible bottom, bottom seams for greater durability, reinforced corners, 25 mil vinyl (25% thicker than most). The deep molded feathertop surface provides reduced surface tension and no pressure points for the greatest comfort and best sleep. The HS502 is a Semi-Waveless waterbed mattress with 2 layers of internal full-body support fiber for a slight wave reduction. Strobel Waterbeds do not require toxic flame retardant chemicals like other mattresses to comply with federal fire safety regulations.