10′ x 15′ – Geotextile Pond Underlayment Kit by H2o Gardens. $54.99. Can also be used on top of the liner if setting heavy or sharp rocks. Underlayment acts as padding for your liner and allows gas to escape. It has a non-slippery surface and provides protection for the liner from rocks and other sharp objects, spreads easily, and saves time compared to using newspaper or sand. Needle-punched to allow trapped gases to escape, not cheap weed barrier!