Congratulations on building your first pond! By now, you've learned that pond builders don't just dig holes, throw in rocks and sand, and start filling with water. Your underlayment and pond liner choices should be durable and fish safe, as everything within the pond will rest over them to prevent water seepage into your yard. Your pond underlayment is a mesh-like material that provides a barrier between your vinyl liner and random knobs or shards on the ground that could puncture the liner. Carlisle AquaTough EPMD .45mil is one of the best pond liners available in the market today. Order pre-selected measurements or contact us for custom special pricing by the square foot.