Pond liners, like swimming pool liners, act as a barrier to keep the water in the pond from leaking into the ground. Pond liners are constructed of polyethylene, which makes them extremely flexible, UV resistant and durable through even the most severe weather and temperatures. Pond liners flexibly conform to the hole you've dug and most come with a 20-year warranty - guaranteeing a leak-proof pond. An underlayment can be used in conjunction with a pond liner to further protect the pond from being punctured and to further prevent risk of leaks. Pond liners are price according to their size and weight. Smallest 8' x 10' liners cost approximately $30 online, while the large 40' x 40' pond liners can cost upwards of $670 online. Pond liners are also sold in rolls, ranging from 5.5' x 100' at $220, up to 50' x 100 for approximately $2,000 online.