Our pond underlayment (underliner) is a tough, thick, nonwoven (non woven) 60 mil geotextile fabric that will protect your liner from any sharp rocks, glass and even against burrowing animals that can damage a liner. Lightweight and easy to use, we recommend and use this thick nonwoven, geotextile underlayment with every pond installation we do. Pond underliner fabric provides insurance for your liner against glass, sharp stones or roots that may have been overlooked during the installation process. Dont take a chance with the backbone of your pond! 72 sq ft.