Firestone 10 x 12 45mil Pond Liner Matching Underlayment Kit Measures 10-feet length by 12-feet width 45 mil Firestone pond liner and matching pond underlayment. Firestone 45mil Pond Liner with 20 Year Warranty - Flexiable, Durable and Fish Safe Matching Non-Woven GeoTextile Pond Underlayment : Non-Woven stable Polypropolyene Fiber Filaments bonded by heatset and needle punched Pond Underlayment has only one job and that is to protect your pond liner. 45 mil Pond liner is very strong and puncture resistant but it does have its limits. Using underlayment can improve the PUNTURE RESISTANCE of your pond liner by 400%. Made in the USA